REESE   LIBRARY 

OK  THK 

UNIVERSITY  OF  CALIFORNIA. 


L 


Accessions  No. 


,  I&Q7- 


THE 


METHOD  OF  DARWIN 


g>tufip  fa  Scientific 


BY 


FRANK    CRAMER 


€p 
ERSITY 
i 


OF  THE 

E 

.-- 


CHICAGO 

A.  C.  McCLURG   AND   COMPANY 
1896 


COPYRIGHT 

BY  A.  C.  MCCLURG  AND  Co. 
A.D.  1806 


TO 

MY    FRIEND    AND    TEACHER, 
BRADFORD   PAUL   RAYMOND. 


PREFACE. 


/TVHIS  attempt  to  analyze  the  method  em- 
*•  ployed  in  the  biological  sciences  arose 
from  the  belief  that  the  direct  study  of  scientific 
method,  as  it  is  illustrated  by  the  works  of  the 
accepted  masters,  is  worthy  of  far  more  careful 
attention  than  is  usually  accorded  to  it.  As  a 
rule,  scientific  men  are  so  deeply  engrossed  in 
their  investigations  that  they  rarely  undertake 
to  discuss  method.  The  logical  processes  in- 
volved and  the  nature  of  the  difficulties  met 
writh  in  scientific  investigation  are  the  same  as 
in  the  practical  affairs  of  life.  The  fundamental 
processes  of  reasoning  are  the  same  everywhere  ; 
and  it  cannot  but  be  helpful  to  study  those  pro- 
cesses as  they  are  actually  applied  by  master 
minds  in  fields  where  precision  of  method  is 
peculiarly  essential.  Even  though  there  may 
be  grave  question  concerning  the  practical  value 
of  the  study  of  formal  logic,  there  can  be  no 


viii  PREFACE. 

question  concerning  the  importance  of  attention 
to  the  best  practice  in  matters  of  reasoning. 

Some  of  the  reasons  which  induced  me  to 
select  Darwin's  works  as  a  basis  for  an  analysis 
of  scientific  method  were:  (i)  the  desire  to 
confine  the  discussion  to  the  writings  of  a  single 
author,  in  order  to  concentrate  the  reader's  at- 
tention upon  a  model;  (2)  the  fact  that  his 
works  cover  a  wide  range  of  subjects,  and  can 
be  read  and  understood  by  those  who  have  had 
only  a  moderate  amount  of  scientific  training; 
and  (3)  above  all,  the  fact  that  Darwin's  inves- 
tigations, and  the  reasoning  based  upon  them, 
have  furnished  the  biological  sciences  with  their 
dominant  principles. 

To  facilitate  the  study  of  his  works  from  the 
point  of  view  of  method,  references  have  been, 
added  to  nearly  all  the  examples  drawn  from 
them.  A  few  examples  have  been  mentioned 
or  briefly  discussed  several  times,  and  this  may 
detract  slightly  from  the  freshness  of  some  parts ; 
but  a  partial  compensation  may  be  found  in  the 
fact  that  the  repetition  of  the  same  example, 
under  different  divisions  of  the  subject,  will 
emphasize  more  strongly  the  complex  inter- 


PREFACE.  ix 

dependence  of  the  various  logical  processes.  In 
several  instances,  particularly  those  of  radicles, 
climbing  plants,  and  electric  organs,  the  discus- 
sions have  been  carried  into  considerable  detail: 
this  has  been  done  for  the  purpose  of  show- 
ing what  an  actual  course  of  investigation  and 
reasoning  is  like,  —  how  results,  whether  true  or 
false,  are  worked  out. 

At  the  same  time,  no  effort  has  been  made  to 
make  the  treatment  of  Darwin's  method  exhaust- 
ive, nor  has  any  formal  explanation  of  the  vari- 
ous logical  processes  been  made.  Those  who 
are  likely  to  read  this  book  are  already  suffi- 
ciently familiar  with  the  terminology  of  logic  and 
the  practice  of  science  to  understand  it  easily; 
and  extended  explanations  would  require  an  ex- 
cursion into  the  domain  of  formal  logic,  which 
is  not  a  part  of  the  purpose  of  the  present 
work. 

Some  of  the  most  important  processes  have 
been  selected,  and  Darwin's  applications  of 
them  illustrated,  in  such  a  way  as  to  confine 
the  whole  discussion  within  the  narrowest  pos- 
sible limits.  It  is  an  easy  matter  to  provoke 
differences  of  opinion  in  discussing  either  the 


X  PREFACE. 

nature  or  the  names  of  the  logical  processes; 
but  as  far  as  it  lay  in  my  power  I  have  avoided 
setting  my  foot  on  controversial  ground.  There 
are  in  the  book,  no  doubt,  many  errors  in 
detail,  but  it  is  hoped  that  no  serious  ones  have 
crept  in. 

I  wish  to  thank  Professor  Francis  Darwin  of 
Cambridge,  England,  and  President  David  Starr 
Jordan  of  Stanford  University,  both  of  whom 
made  important  suggestions,  and  Professor  H. 
B.  Lathrop  of  Stanford  University,  who  care- 
fully revised  the  manuscript. 

FRANK  CRAMER. 
MANZANITA  HALL. 


CONTENTS. 


CHAPTER  PAGE 

I.    EDUCATION  AND  THE  ART  OF  REASONING  .    .  15 

II.    DARWIN'S  VIEW'S  OF  METHOD 25 

III.  STARTING    POINTS.  —  METHOD    OF    STARTING 

INVESTIGATIONS.  —  ISOLATED  PHENOMENA  .  47 

IV.  EXHAUSTIVENESS. — TlME  GIVEN  TO   INVESTI- 

GATIONS.—  TREATMENT  OF  OBJECTIONS  .     .  60 

V.    NEGATIVE  EVIDENCE 82 

VI.    CLASSIFICATION 88 

VII.    ANALOGY 95 

VIII.    INDUCTION 107 

IX.  DEDUCTION.  —  EXPLANATION  OF  KNOWN  FACTS. 
—  IMPORTANCE  OF-  THEORY  TO  GOOD  OB- 
SERVATION    121 

X.    DEDUCTION.  —  ANTICIPATION 133 

XI.    DEDUCTION.  — GENERAL  INSTANCES      .    .    .    .  160 

XII.    UNVERIFIED  DEDUCTIONS 178 

XIII.  ERRONEOUS  DEDUCTION 192 

XIV.  GENERAL  DISCUSSIONS     .    . 207 

XV.    LOGICAL  HISTORY  OF  THE  PRINCIPLE  OF  NAT- 
URAL SELECTION 212 

XVI.    CONCLUSION 229 


LIST   OF    DARWIN'S    WORKS    REFERRED    TO 
IN    THE    FOOT-NOTES. 

Origin  of  Species,  Sixth  Edition,  1872. 

Descent  of  Man  and  Selection  in  Relation  to  Sex,  1871. 

Variation  of  Animals  and  Plants  under  Domestication, 
Second  Edition,  1875. 

Expression  of  the  Emotions  in  Man  and  the  Lower  Ani- 
mals, 1872. 

A  Monograph  of  the  Cirripedia,  Vol.  I.,  1851  ;  Vol.  II., 
1854- 

The  Formation  of  Vegetable  Mould  through  the  Action 
of  Earthworms,  1881. 

Naturalist's  Voyage  around  the  World,  1860. 

Geological    Observations    on  Volcanic    Islands   and    on 

Parts  of  South  America,  Second  Edition,  1876. 
Structure  and  Distribution  of  Coral  Reefs,  Third  Edition, 


Fertilization  of  Orchids,  Second  Edition,  1877. 

Effects  of  Cross-  and  Self-Fertilization,  1876. 

Different  Forms  of  Flowers  on  Plants  of  the  same  Species, 

1877- 

Insectivorous  Plants,  1875. 
Climbing  Plants,  1875. 
Power  of   Movement  in  Plants,  by  Charles  and  Francis 

Darwin,  1881. 
Life  and  Letters  of  Charles  Darwin,  by  Francis  Darwin, 

two  vols.,  1887. 


A  SCIENTIFIC  discovery  is  the  outcome  of  an  interesting 
process  of  evolution  in  the  mind  of  its  author.  When  we  are 
able  to  detect  the  germs  of  thought  in  which  such  a  discovery 
has  originated,  and  to  trace  the  successive  stages  of  the  rea- 
soning by  which  the  crude  idea  has  developed  into  an  epoch- 
making  book,  we  have  the  materials  for  reconstructing  an 
important  chapter  of  scientific  history.  —  PROF.  J.  W.  JUDD, 
Critical  Introduction  to  Bettany's  Edition  of  Darwin's  "Structure 
and  Distribution  of  Coral  Reefs" 


UNIVERSITY 


THE   METHOD   OF   DARWIN. 


i. 

EDUCATION  AND   THE  ART  OF   REASONING. 

TT  is  an  opinion  not  uncommon  among  educa- 
tors that  a  definition  of  education  which 
would  cover  all  kinds  of  training  is  an  impossi- 
bility. Such  a  definition,  as  it  widens  for  the 
reception  of  manual  training  and  the  study  of 
Greek,  kindergarten  work  and  the  post-graduate 
course,  certainly  threatens  to  become  "  like  a 
circle  in  the  water,  which  never  ceaseth  to 
enlarge  itself  till  by  broad  spreading  it  dis- 
perse to  naught."  The  difficulty  in  framing  it 
ha's  apparently  increased  in  recent  years,  since 
the  old  standards  of  value  in  education  have 
had  to  struggle  for  existence  with  all  the  other 
college  and  university  studies.  The  old  defini- 
tion, "to  lead  out  and  train  the  mental  powers," 
is  comprehensive  enough  for  all  purposes,  for 
it  tells  neither  what  the  mental  powers  to  be 
trained  are,  nor  how  they  are  to  be  trained ; 


1 6  THE  METHOD   OF  DARWIN. 

by  the  change  of  a  word  or  two  it  would  apply 
equally  well  to  the  art  of  breaking  colts.  If 
the  definition  is  stated  more  in  detail,  it  is 
found  to  lie  entirely  in  the  domain  of  applied 
logic;  and  education  of  the  intellect,  in  the 
only  sense  in  which  it  can  cover  the  whole 
field,  is  the  process  of  training  the  intellect  in 
the  art  of  reasoning. 

If  there  is  or  ever  shall  be  a  common  aim  in 
all  phases  of  education,  it  will  be  based  upon 
this  common  element.  However  important  the 
information  may  be  which  is  conveyed  to  the 
student  in  any  department  of  study,  his  ability 
to  retain  it  and  use  it  for  further  acquisition 
depends  entirely  on  the  method  by  which  he 
acquired  it,  and  the  degree  to  which  he  has  be- 
come master  of  that  method.  The  facts  neces- 
sary for  a  new  investigation  are  easily  brought 
to  hand  if  the  intellect  has  been  trained  to 
work  independently.  One  of  the  most  strik- 
ing things  in  Darwin's  Autobiography  is  the 
relative  importance,  to  be  mentioned  again 
hereafter,  which  he  assigns,  in  his  analysis 
of  his  own  education,1  to  the  accumulation  of 
facts  and  to  the  development  of  mental  habits. 
Probably  few  minds  ever  possessed  in  a  higher 

1  Life  and  Letters  of  Charles  Darwin,  by  Francis  Darwin, 
Vol.  I.  pp.  51,  52. 


EDUCATION  AND   ART  OF  REASONING.      I/ 

degree  the  power  to  collect  and  utilize  facts. 
He  said  the  real  education  of  his  mind  began 
on  the  Beagle  voyage.  And  yet  he  gave  a 
very  subordinate  place  to  the  vast  number  of 
facts  with  which  he  became  acquainted  on  the 
voyage,  and  assigned  supreme  importance  to 
the  habits  of  incessant  industry  and  concen- 
trated attention  which  were  developed  in  him, 
and  to  the  necessity  of  reasoning  in  the  solution 
of  geological  problems. 

If  the  most  important  and  only  common  ele- 
ment in  education  is  the  development  of  the 
power  of  reasoning,  it  may  seem  strange  that 
logic,  proudly  called  the  science'  of  sciences, 
should  play  so  obscure  a  role  that  in  many 
institutions  it  is  practically  ignored,  and  in 
the  rest  it  is  tolerated  in  a  very  brief  course. 
The  two  most  probable  reasons  for  this  are, 
first,  the  general  notion  that  the  human  mind 
learns  to  reason  as  the  human  body  learns  to 
walk,  that  there  is  no  need  of  teaching;  that 
as  training  in  the  latter  can  only  produce  a 
Delsartean  gait,  which  for  practical  purposes  is 
little  superior  to  an  awkward  wabble,  so  train- 
ing in  the  art  of  reasoning  is  likely  to  produce 
nothing  but  over-refinement,  which  accepts 
indifferently  postulates  foolish  and  wise,  and 
seeks  only  to  draw  out  their  consequences  into 
2 


I fr  THE  METHOD   OF  DARWIN. 

gossamer  threads.  The  second  reason  is,  that 
the  logic  usually  taught  is  not  the  logic  of 
common  life  refined  by  successful  scientific 
experience,  but  "formal  logic,"  emptied  of  all 
-— ^  contents  and  divested  of  all  covering.  Every 
fool  can  walk,  and,  as  Darwin  truly  said,  any 
fool  can  generalize  and  speculate.  But  the 
secret  of  originality,  ingenuity,  skill  to  seize 
facts,  grasp  their  significance,  and  anticipate 
consequences,  is  not  hidden  here.  Not  the 
power  to  reason,  but  the  power  to  reason 
quickly  and  unerringly  and  doggedly  and  im- 
partially is  the  basis  of  success  alike  for  the 
business  man  and  for  the  man  of  science. 

If  skilful  and  accurate  reasoning  constitutes 
so  essential  an  element  of  education,  and  if 
logic,  as  formally  taught,  is,  for  the  mass  of 
students,  so  barren  of  results,  it  is  pertinent 
to  inquire  what  provision  for  logical  training 
is  made  in  the  general  instruction  of  colleges 
and  universities.  In  the  evolution  of  the  col- 
lege curriculum  the  individuality  of  the  student 
has  finally  been  recognized  and  provided  for, 
and  the  dignity  of  the  sciences,  as  subjects 
conducive  to  mental  discipline,  has  become  an 
accepted  fact.  The  material  of  education  has 
by  this  been  both  increased  and  improved. 
Method  has  also  undergone  profound  changes. 


EDUCATION  AND  ART  OF  REASONING.       K) 

The  laboratory  for  science,  sources  of  informa- 
tion for  history,  inventional  work  in  mathe- 
matics, all  bear  witness  that  the  student  has 
been  brought  into  direct  contact  with  the  mate- 
rial by  means  of  which  his  intellect  is  to  be 
trained.  The  best  laboratory  hand-books  are 
no  longer  books  of  directions,  but  of  sugges- 
tion and  question ;  these  books  constitute  a 
distinct  recognition  that  the  art  of  reasoning  is 
the  heart  of  education,  that  the  true  student  is 
from  first  to  last  a  discoverer,  and  that  any 
method  which  makes  the  discoveries  for  him  is 
wrong. 

There  are,  however,  two  very  distinct  orders 
of  reasoning:  the  order  of  discovery,  which  the 
mind  follows  as  it  winds  its  way  among  facts, 
adopting  tentatively  hypotheses  which  are  after- 
wards rejected,  and  groping  along  the  border 
of  the  unknown  in  the  pursuit  of  knowledge; 
and  the  order  of  proof  or  argument,  used  by 
the  investigator  in  his  effort  to  convince  his 
hearer  or  reader  of  the  truth  of  his  results. 
The  order  of  proof  may  ignore  entirely  the 
steps  by  which  the  discoveries  were  made,  the 
materials  collected,  and  the  conclusions  drawn. 
The  aim  is  conviction,  and  the  evidence  is 
arranged  in  the  most  lucid  order  to  support  the 
conclusions  established  at  the  end  of  an  inves- 


2O  THE  METHOD   OF  DARWIN. 

tigation.  It  is  true  that  sometimes  the  order 
of  discovery  is  the  best  for  purposes  of  proof; 
but  unless  it  happen  to  be  so,  it  is  ignored 
after  the  investigation  is  completed. 

Logicians  insist  that  their  science  is  the 
science  of  proof ;  that  it  does  not  furnish  truth, 
but  tests  by  which  to  determine  whether  or  not 
a  supposed  discovery  is  truth.  Books  and  lec- 
tures are  invariably  built  up  on  the  plan  of 
proof.  In  them  the  question  how  a  conclusion 
was  reached  is  rarely  presented,  and  when  it 
does  occur,  pains  is  seldom  taken  to  provide 
for  its  answer.  So  far,  then,  as  these  ele- 
ments of  education  are  concerned,  the  student 
is  made  a  recipient.  He  is  struck  by  the  lucid 
arrangement  of  facts,  the  majestic  sweep  of  the 
argument,  and  wonders  why  the  world  did  not 
sooner  get  hold  of  truth  that  seems  so  conclu- 
sive to  him.  In  the  laboratory,  the  hand-books" 
tell  him  what  to  look  for  and  where  to  find  it ; 
and  in  the  lecture-room  the  facts  are  arranged 
and  the  theoretical  explanations  are  made  for 
him.  Thus  in  neither  of  the  two  practical 
divisions  of  the  art  of  reasoning  is  he  allowed 
to  follow  even  the  untrained  impulses  of  his 
intellect.  The  average  student  knows  next  to 
nothing  of  the  science  of  reasoning,  and  is 
largely  prevented  from  practising  the  art  of 


EDUCATION  AND   ART  OF  REASONING.     21 

reasoning  either  from  the  standpoint  of  dis- 
covery or  from  that  of  proof. 

This  general  indictment  against  the  college 
curriculum  needs  to  be  hedged  in  by  many 
qualifications;  but  there  are  none  which  seri- 
ously break  its  force.  The  graduate  student  is 
left  largely  to  his  own  resources,  and  must  do 
his  own  reasoning  or  go  without  any.  A  few 
of  the  best  laboratory  hand -books  question  and 
suggest  to  the  student,  so  that  he  is  kept  from 
dissipating  his  energy;  but  they  largely  compel 
him  to  make  his  own  discoveries  and  develop 
independence. '  A  few  teachers  habitually,  and 
many  teachers  occasionally,  compel  their  stu- 
dents to  cast  their  materials  into  the  order  of 
proof  or  argument  in  topical  reports ;  but  those 
very  reports  are,  as  a  rule,  the  best  illustra- 
tion of  the  utter  lack  of  logical  insight  on  the 
part  of  students. 

Before  applied  logic  secures  general  recog- 
nition proportionate  to  its  importance,  it  will 
have  to  demonstrate  its  ability  to  lay  in  the 
mind  of  the  student  the  foundation  of  accurate 
thinking,  to  furnish  him  with  analyses  of  mod- 
els of  successful  reasoning,  and  criteria  by  which 
to  detect  false  reasoning. 

It  is  strange  that  while  the  study  of  our 
mother  tongue  and  of  all  the  sciences  has 


22  THE  METHOD   OF  DARWIN; 

undergone  so  great  a  revival,  the  science  of 
reasoning  should  still  be  lying  in  the  valley  of 
dry  bones.  Fate  may  have  decreed  that  its 
revival  should  be  the  crowning  phase  of  modern 
progress  in  educational  methods.  Slowly  but 
very  surely  these  methods  are  drifting  in  the  di- 
rection of  a  more  extended  and  a  more  profound 
recognition  of  the  importance  of  reasoning. 

When  educational  practice  shall  have  demon- 
strated the  importance  of  the  art  of  reasoning, 
scientific  models  of  the  art,  for  purposes  of  logi- 
cal study,  will  be  found  to  be  rare,  especially 
those  that  reveal  the  order  of  discovery.  The 
scientist,  after  establishing  a  conclusion  to  his 
own  satisfaction,  is  not  concerned  with  telling 
other  people  how  he  reached  it,  but  with  con- 
vincing them  of  its  truth.  For  this  purpose 
he  throws  his  conclusions  and  facts  into  the. 
order  best  suited  to  form  a  compact  argument. 
In  the  vast  majority  of  cases  it  is  impossible  to 
follow  out  the  original  course  of  thought  by  a 
study  of  the  results  as  they  are  embodied  in  a 
book.  It  would  probably  be  difficult  for  an 
author  himself  to  trace  again  the  windings  of 
his  thought.  Therefore,  while  there  is  a  fair 
number  of  models  for  the  study  of  argument, 
the  writers  who  habitually  take  their  readers 
so  far  into  their  confidence  as  to  tell  them  by 


EDUCATION  AND  ART  OF  REASONING.     2$ 

what  steps  they  arrived  at  facts  and  conclusions, 
are  extremely  rare. 

Several  reasons  have  led  to  the  following 
study  of  Darwin's  method:  first,  conviction  of 
the  supreme  practical  importance  of  the  direct 
study  of  scientific  method;  secondly,  the  fact 
that  logicians  and  scientific  philosophers  draw 
their  illustrations  of  scientific  method  almost 
exclusively  from  the  physical  sciences;  thirdly, 
while  those  illustrations  are  fascinating  on 
account  of  their  brilliancy  and  their  approach 
toward  mathematical  certainty,  the  biological 
sciences  are  much  better  adapted  to  furnish 
models  for  the  average  student,  because  in  the 
nature  of  their  logical  difficulties  they  approach 
more  nearly  to  the  experiences  of  common 
life;  fourthly,f Darwin's  custom  of  presenting 
all  sides  of  a  case  very  frequently  led  him  to 
expose  the  original  course  of  his  thought  and 
the  order  of  his  discoveries  so  clearly  as  to 
make  the  reader  almost  feel  that  he  and  Dar- 
win are  making  the  discovery  together.  Dar- 
win consciously  recognized  or  unconsciously 
felt  that  there  was  considerable  power  to  pro- 
duce conviction  in  an  understanding  of  the 
particular  way  in  which  the  truth  was  first 
reached.  He  so  habitually  took  the  reader 
into  his  confidence  that  he  will  probably  always 


24  THE  METHOD   OF  DARWIN. 

remain   the  clearest    model    in    the  biological 
world  for  the  study  of  applied  lo^kT^) 

There  are  two  ways  open  for  the  logical 
study  of  Darwin's  works:  one  in  which  the 
methods  of  handling  material  and  the  differ- 
ent logical  processes  would  be  illustrated  by 
examples  from  different  parts  of  his  works; 
and  the  other  in  which  each  of  his  investiga- 
tions would  be  studied  apart  from  the  rest  for 
the  purpose  of  noting  the  part  which  induc- 
tion, deduction,  analogy,  and  verification  play 
in  producing  the  results.  The  former  was 
chosen,  because  in  that  way  the  best  examples 
of  each  of  the  logical  processes  could  be 
brought  together  in  the  smallest  compass, 
while  the  latter  would  require  an  elaborate 
summary  of  the  works  themselves  before  their 
logic  could  be  discussed.  No  epitome,  much 
less  selected  examples,  can  be  made  a  substi- 
tute for  a  logical  study  of  the  works  them- 
selves; it  can  at  most  serve  as  a  guide  to 
further  study. 


II. 

DARWIN'S   VIEWS   OF   METHOD. 

IT  is  necessary  to  inquire  briefly  into  Dar- 
win's own  views  of  scientific  method. 
He  has  given  us  the  data  for  the  inquiry,  both 
in  direct  statements  and  indirectly  by  his 
opinions  of  the  work  and  ability  of  other  men. 
In  connection  with  this  inquiry  must  be  con- 
sidered the  intellectual  and  moral  qualities  of 
the  man  himself,  and  the  external  influences 
that  bore  upon  him. 

In  some  quarters  the  notion  is  entertained 
that  the  scientific  method  leads  infallibly  to 
the  truth,  and  that  it  is  something  quite  dis- 
tinct from  the  logical  method  of  every-day 
life;  and  yet  there  is  a  general  haziness  as  to 
what  the  scientific  method  is.  The  aim  of  the 
scientist  is  truth,  but  he  has  no  special  mental 
faculty  with  which  to  discover  scientific  truth. 
He  approaches  his  problem,  equipped  with  the 
same  logical  processes  that  the  common  man 
uses  in  arriving  at  facts  that  are  important  to 
his  success  in  life.  Neglect  in  applying  those 


26  THE  METHOD  OF  DARWIN. 

processes  strictly,  in  either  case,  results  in 
failure,  and  fidelity  to  them  is  the  measure  of 
success.  There  is  something  more  than  merely 
a  weak  analogy  between  the  very  small  pro- 
portion of  successful  business  men  and  the 
similarly  small  proportion  of  really  successful 
creative  scientific  men.  The  failures  on  both 
sides  are  clue  to  the  same  kind  of  intellectual 
errors.  Nor  are  the  principles  of  the  scien- 
tific method  less  clearly  understood  by  success- 
ful business  men  than  by  successful  scientific 
investigators. 

Darwin  has  said  that  the  training  which  he 
got  on  the  Beagle  voyage  was  the  first  real  ed- 
ucation of  his  mind.1  His  University  course 
at  Cambridge  had  been  mechanical.  He  de- 
clared that  the  study  of  Paley's  ''Evidences" 
and  "Natural  Theology  "  gave  him  as  much, 
delight  as  did  Euclid.2  And  he  believed  at 
the  time,  and  to  the  end  of  his  life,  that  the 
study  of  these  works  was  the  only  part  of  his 
academic  course  that  contributed  to  the  educa- 
tion of  his  mind.  He  got  no  inspiration,  and 
very  little  knowledge,  from  his  medical  course 
at  Edinburgh ;  one  of  the  principal  records  of 
that  course  is  his  opinion  that  there  are  no 

1  Life  and  Letters,  Vol.  I.  pp.  51,  52. 

2  Ibid.,  p.  41. 


DA  R  WIN'S  VIE  WS   OF  ME  TIIOD.  2  / 

advantages  and  many  disadvantages  in  lectures 
compared  with  reading.1 

He  has  given  in  his  Autobiography  the  vari- 
ous special  subjects  that  occupied  his  attention 
on  the  Beagle  voyage.  He  attended  to  Zool- 
ogy, Botany,  and  Geology.  After  mentioning 
the  others,  he  said  that  Geology  "  was  far  more 
important,  as  reasoning  here  comes  into  play. 
On  first  examining  a  new  district,  nothing  can 
appear  more  hopeless  than  the  chaos  of  rocks; 
but  by  recording  the  stratification  and  nature 
of  the  rocks  and  fossils  at  many  points,  always 
reasoning  and  predicting  what  will  be  found 
elsewhere,  light  soon  begins  to  dawn  on  the 
district,  and  the  structure  of  the  whole  becomes 
more  or  less  intelligible."2  It  is  interesting 
to  recall  that  he  finally  committed  the  shooting 
of  birds  to  his  servant  in  order  that  he  might 
devote  himself  to  the  geology  of  the  districts 
in  which  he  worked.  The  zoological  and 
botanical  materials  which  he  collected  were 
largely  worked  up  by  other  scientists  after  his 
return.  On  these  subjects  he  collected  a  vast 
amount  of  information,  which  gave  birth  to 
his  great  biological  theories,  and  was  indis- 
pensable in  the  work  of  his  later  life.  But  he 

1  Life  and  Letters,  Vol.  I.  p.  33. 

2  Ibid.,  pp.  51,  52. 


28  THE  METHOD   OF  DARWIN. 

did  not  work  this  information  into  a  system 
and  bend  his  energies  upon  it.  He  devoted 
himself  to  the  solution  of  .geological  problems 
in  the  field;  while  the  biological  problems 
only  arose  in  their  early  shadowy  outlines 
during  the  voyage,  and  remained  in  the  form 
of  questions  till  long  after  his  return.  Thus 
it  came  about  that  from  an  educational  point 
of  view  his  biological  work  was  secondary  and 
the  geological  work  pre-eminent.  Had  his 
life-work  ended  with  the  reports  of  the  Beagle 
voyage,  he  would  have  been  rated  as  a  geol- 
ogist. But  after  his  return  he  exercised  upon 
great  biological  problems  the  intellectual 
strength  and  vigor  which  had  been  developed 
by  the  solution  of  geological  problems  in  the 
field.  "  The  above  various  special  studies  were, 
however,  of  no  importance,"  he  said,  "compared 
with  the  habit  of  energetic  industry  and  of  con- 
centrated attention  to  whatever  I  was  engaged 
in  which  I 'then  acquired.  Everything  about 
which  I  thought  or  read  was  made  to  bear 
directly  on  what  I  had  seen  or  was  likely  to 
see."  He  had  problems  constantly  before  him, 
and  the  time  allowed  for  their  solution  was 
always  limited  by  his  own  movements  and 
those  of  the  ship;  so  that  energy  and  concen- 
tration became  habitual  in  a  mind  already 


DARWIN'S   VIEWS   OF  METHOD.  29 

strong  by  nature.  His  isolation  from  other 
scientific  men  and  from  books  doubtless  also 
developed  in  him  the  habit  of  using  all  the 
facts  that  presented  themselves,  and  directly 
and  indirectly  getting  at  their  significance. 
He  could  not  lay  his  hands  on  ready-made 
explanations  of  the  facts  that  came  before  him, 
and  was  compelled  to  explain  them  himself. 
He  said  of  himself,  "  I  think  I  am  superior  to 
the  common  run  of  men  in  noticing  things 
which  easily  escape  attention,  and  in  observ- 
ing them  carefully.  .  .  .  From  rny  earliest 
youth  I  have  had  the  strongest  desire  to  under- 
stand or  explain  whatever  I  observed,  —  that 
is,  to  group  all  facts  under  some  general  laws."  1 
These  natural  traits  were  of  necessity  strength- 
ened and  developed  by  their  incessant  exercise 
on  the  voyage. 

Another  prominent  trait  in  Darwin  was  the 
accuracy  with  which  he  made  his  observations 
and  experiments.  "  He  saved  a  great  deal  of 
time  through  not  having  to  do  things  twice." 
And  he  always  "wished  to  learn  as  much  as 
possible  from  an  experiment,  so  that  he  did 
not  confine  himself  to  observing  the  single 
point  to  which  the  experiment  was  directed, 
and  his  power  of  seeing  a  number  of  things 

1  Life  and  Letters,  Vol.  I.  p.  83. 


UNIVEHSIT 


30  THE  METHOD   OF  DARWIN. 

was  wonderful."  1  Both  his  accuracy  of  obser- 
vation and  his  grasp  of  all  the  facts  connected 
with  an  experiment  were  doubtless  made  hab- 
itual on  the  voyage  by  the  never  absent  con- 
sciousness that  there  was  only  one  opportunity 
to  do  whatever  he  did. 

During  that  memorable  voyage  Darwin's 
education  went  on,  unhampered  by  laboratory 
hand-books  with  directions  for  finding  the 
facts,  or  by  professors  to  do  the  reasoning  for 
him  either  before  or  after  the  facts  were  found. 
In  all  his  work  there  was  the  complex  and 
incessant  interplay  of  observation,  induction, 
deduction,  and  verification  which  constitutes 
the  scientific  method.  The  necessity  of  work- 
ing rapidly,  accurately,  and  thoroughly  forced 
upon  him  by  the  consciousness  that  his  time 
was  limited,  and  that  the  work  could  not  be 
done  over  again,  coupled  with  his  native  energy 
and  independence,  accounts  for  the  character 
and  quantity  of  his  scientific  work. 

The  mental  traits  alluded  to  were  coupled 
with  remarkable  conscientiousness.  In  the 
long  run  it  pays  the  scientist  to  be  honest,  not 
only  by  not  making  false  statements,  but  by 
giving  full  expression  to  facts  that  are  opposed 
to  his  views.  Moral  slovenliness  is  visited 

1  Life  and  Letters,  Vol.  I.  pp.  121,  122. 


DARWIN'S    VIEWS   OF  METHOD.  31 

with  far  severer  penalties  in  the  scientific  than 
in  the  business  world.  Scientific  results  are 
used  as  foundations  for  further  investigations, 
and  for  this  reason  they  are  tested  again  and 
again;  and  if  any  man's  work  is  unreliable 
it  is  done  over  by  some  one  else,  who  reaps 
the  permanent  credit.  But  the  temptations  to 
make  statements  too  broad,  to  neglect  objec- 
tions, to  smooth  over  difficulties  superficially, 
are  almost  infinite.  There  is  apparent  through- 
out all  of  Darwin's  work  much  more  than  the 
intellectual  uprightness  that  is  due  to  a  belief 
in  "  reward  and  punishment. "  The  very  grain  of 
his  scientific  character  was  conscientiousness. 

His  educational  history,  his  thoroughness,  his 
scientific  honesty,  his  logical  power,  his  power 
of  minute  observation  and  broad  generaliza- 
tion, the  greatness  of  the  problems  with  which 
he  dealt,  and  the  profound  influence  of  his 
views  upon  the  thought  of  the  world,  all  con- 
spire to  make  him  a  model  in  the  study  of 
scientific  method.  Some  of  his  views  have 
been  rejected,  and  many  may  be  profoundly 
modified  by  more  accurate  knowledge,  but 
these  things  will  in  no  way  affect  the  value 
of  Darwin  as  a  type  of  what  education  should 
accomplish,  and  how  it  must  accomplish  it. 

Darwin  appreciated  the   humblest  scientific 


32  THE  METHOD   OF  DARWIN. 

work,  and  listened  with  deference  to  the  sug- 
gestions of  others.  He  never  dealt  out  primi- 
tive justice  to  his  opponents  on  the  principle 
of  an  eye  for  an  eye  and  a  tooth  for  a  tooth. 
He  is  morally  famous  for  the  forbearance  that 
he  exercised  toward  those  who  attacked  him. 
This  fame  is  heightened  by  the  fact  that  he 
was  an  acute  judge  of  mental  quality.  His 
investigations  made  it  necessary  for  him  to 
collect  information  from  all  sorts  of  sources, 
—  not  only  at  first  hand,  from  Nature  herself, 
but  at  second  hand,  from  many  kinds  of  books, 
made  by  many  kinds  of  men.  He  has  com- 
plained that  it  was  exceedingly  difficult  to  find 
out  what  and  whom  to  trust.1  But  his  criti- 
cisms of  certain  kinds  of  work  show  how 
definite  were  his  standards  of  value  in  measur- 
ing scientific  results. 

Perhaps  the  most  savage  things  Darwin  ever 
wrote  are  contained  in  letters  to  Hugh  Strick- 
land, and  relate  to  the  nomenclature  of  sys- 
tematic zoology  and  botany.2  He  expressed 
freely  his  contempt  for  describers  of  species 
who  think  the  honor  consists  in  having  one's 
name  appended  to  that  of  a  newly  described 
species,  and  whose  work  is  generally  so  inac- 

1  Life  and  Letters,  Vol.  II.  p.  75. 

2  Ibid.,  Vol.  I.  pp.  334,  338>  344- 


DARWIN'S    VIEWS  OF  METHOD.  33 

curate  or  imperfect,  or  both,  as  to  be  practi- 
cally worthless  for  any  of  the  higher  purposes 
of  science.  Nor  was  he  satisfied  with  mere 
details  even  when  they  were  accurate.  In  his 
reminiscences  of  Robert  Brown  he  said  that 
Brown  seemed  to  him  "to  be  chiefly  remark- 
able for  the  minuteness  of  his  observations, 
and  their  perfect  accuracy."1  Darwin  often 
took  breakfast  with  Brown,  and  on  those  occa- 
sions the  latter,  according  to  Darwin,  "poured 
forth  a  rich  treasure  of  curious  observations 
and  acute  remarks;  but  they  almost  always 
related  to  minute  points,  and  he  never  with 
me  discussed  large  or  general  questions  of 
science." 

He  has  given  us  an  interesting  example  of 
his  opinion  of  the  opposite  tendency  toward 
speculation,  to  the  neglect  of  facts.  During 
his  career  as  a  medical  student  he  admired 
greatly  his  grandfather  Erasmus  Darwin's 
"  Zoonotnia."2  "But  on  reading  it  a  second 
time,"  he  said,  "after  an  interval  of  ten  or 
fifteen  years,  I  was  much  disappointed,  the 
proportion  of  speculation  being  so  large  to  the 
facts  given."  These  criticisms  of  the  work 
and  methods  of  others  are  in  perfect  accord 

1  Life  and  Letters,  Vol.  I.  pp.  57,  6b. 


34  THE  METHOD   OF  DARWIN. 

with  his  own  practice.  He  combined  in  him- 
self the  qualities  of  both  Brown  and  his  own 
grandfather.  His  works  are  a  series  of  models 
of  the  scientific  method,  because  of  the  rare 
and  happy  combination  of  minute  and  accurate 
observation  and  daring  speculation  followed  by 
ruthless  testing  and  pruning  of  his  hypotheses. 
He  thought  it  worth  while  to  notice  and  pene- 
trate into  the  meaning  of  the  most  insignifi- 
cant fact,  and  was  capable  of  sweeping  the 
whole  earth  for  evidence  in  support  of  his 
largest  theories.  He  could  take  the  time  to 
count  twenty  thousand  seeds  of  Ly thrum  sali- 
caria; 1  and  his  prophetic  philosophical  eye 
led  him  to  exclaim,  "What  a  science  Natural 
History  will  be  when  we  are  in  our  graves, 
when  all  the  laws  of  change  are  thought  one  of 
the  most  important  parts  of  Natural  History !  ".2 
At  various  times  and  under  various  circum- 
stances Darwin  expressed  fragmentary  opinions 
concerning  what  constitutes  scientific  method, 
but  never  tried  to  make  a  complete  statement  of 
it.  His  notion  of  what  the  method  is,  is  shown 
mostly  by  what  he  said  concerning  "deduc- 
tion." For  instance,  concerning  Bastian's 
work,  he  said,  in  a  letter  to  Wallace,  "I  am 

1  Different  Forms  of  Flowers,  etc.,  p.  189. 

2  Life  and  Letters,  Vol.  I.  p.  439. 


DARWIN'S   VIEWS  OF  METHOD.  35 

not  convinced,  partly,  I  think,  owing  to  the 
deductive  cast  of  much  of  his  reasoning;  and 
I  know  not  why,  but  I  never  feel  convinced  by 
deduction,  even  in  the  case  of  H.  Spencer's 
writings";1  and  in  a  letter  to  John  Fiske,  "I 
find  that  my  mind  is  so  fixed  by  the  inductive 
method,  that  I  cannot  appreciate  deductive 
reasoning;  I  must  begin  with  a  good  body  of 
facts,  and  not  from  principle  (in  which  I  always 
suspect  some  fallacy),  and  then  as  much  deduc- 
tion as  you  please."  2 

Now  deduction  means,  in  one  of  its*  senses, 
reasoning  from  the  general  to  the  particular, 
from  a  law,  principle,  or  general  fact  to  a 
particular  fact.  But  in  the  above  quotations 
Darwin  meant  by  deduction,  and  the  deductive 
method,  reasoning  from  postulates  the  truth  of 
which  is  accepted  as  beyond  dispute.  Induc- 
tion, as  a  logical  process,  means  reasoning 
from  particular  to  general,  from  facts  to  laws 
or  principles.  But  induction,  or  inductive 
method,  when  used  in  a  sense  synonymous 
with  scientific  method,  includes  all  the  logical 
processes,  induction,  deduction,  analogy,  veri- 
fication, —  every  way  in  which  the  intellect 
passes  from  fact  to  fact.  This  is  widely  differ- 
ent from  what  Bacon  originally  meant  by  induc- 

1  Life  and  Letters,  Vol.  II.  p.  346.  2  Ibid.,  p.  371. 


36  THE  METHOD   OF  DARWIN. 

tive  method;  but  practically  no  scientific  man 
has  ever  followed  Bacon's  method. 

The  inductive  method,  as  illustrated  by  Dar- 
win's own  work,  and  as  understood  by  all  who 
think  clearly  on  the  subject,  consists  in  the 
formation  of  an  hypothesis  from  the  facts  by 
induction  at  the  earliest  possible  moment  in 
an  investigation,  deductive  application  of  the 
hypothesis  to  known  facts,  and  in  the  search 
for  others  that  ought  to  exist  if  it  is  true,  until 
it  proves  itself  imperfect.  By  the  help  of  the 
new  facts  the  hypothesis  is  improved  (by  in- 
duction) and  again  applied,  until  by  successive 
approximations  it  reaches  the  truth.  So  that 
in  the  so-called  inductive  or  scientific  method 
deduction  is  far  more  extensively  used  than 
induction.  But  to  say  that  one  of  the  processes 
is  more  important  than  the  other  would  be  like 
saying  that  the  female  element,  for  example, 
is  more  important  for  reproduction  than  the 
male  element.  It  is  interesting  to  note  in 
this  connection  that  John  Stuart  Mill,  the 
modern  logician  who  has  stood  out  as  the 
champion  of  the  inductive  method,  has  incon- 
sistently described  the  combination  "hypoth- 
esis, deduction,  and  verification,"  as  the 
deductive  method.1 

1  Mill,  System  of  Logic,  People's  Edition,  p.  304. 


DARWIN'S   VIEWS  OF  METHOD.  37 

The  scientific  or  inductive  method  as  under- 
stood and  practised  by  Darwin  begins  and 
ends  with  facts.  It  takes  nothing  for  granted 
that  relates  to  the  matter  under  investigation, 
and  assumes  as  true  only  such  things  as  the 
law  of  causation  and  the  validity  of  the  reason- 
ing processes;  while  the  deductive  method,  as 
understood  by  him,  starts  from  principles 
whose  truth  is  not  questioned.  Regarded 
simply  as  a  logical  process,  however,  deduc- 
tion is  equally  valid  whether  the  premises  are 
assumed  to  be  true  or  admitted  to  be  hypo- 
thetical. 

The  ideal  attitude  of  the  scientific  mind  is 
beautifully  described  in  Darwin's  own  words 
concerning  himself:  "I  have  steadily  endeav- 
ored to  keep  my  mind  free  so  as  to  give  up  any 
hypothesis,  however  much  beloved,  (and  I  can- 
not resist  forming  one  on  every  subject,)  as 
soon  as  facts  are  shown  to  be  opposed  to  it. 
Indeed,  I  have  had  no  choice  but  to  act  in  this 
manner,  for,  with  the  exception  of  the  Coral 
Reefs,  I  cannot  remember  a  single  first -formed 
hypothesis  which  had  not  after  a  time  to  be 
given  up  or  greatly  modified.  This  has  natu- 
rally led  me  to  distrust  greatly  deductive 
reasoning  in  the  mixed  sciences."1  *j>  ^ 

1  Life  and  Letters,  Vol.  I.  p.  83. 


38  THE  METHOD   OF  DARWIN. 

It  has  been  said  that  deduction,  regarded 
merely  as  a  logical  process,  is  equally  valid 
whether  the  premises  are  assumed  to  be  true 
or  are  admitted  to  be  hypothetical.  If  the 
premises  are  true,  the  conclusion  must  be  true. 
What  brought  the  old  deductive  process  into 
so  general  disrepute  was  not  its  inadequacy, 
but  the  using  as  premises  in  the  process  prin- 
ciples that  were  held  to  be  beyond  dispute. 
To  this  end  the  a  priori  reasoner  went  farther 
and  farther  back  in  his  philosophical  repertory, 
until  he  reached  principles  or  axioms  that  he 
felt  could  not  be  denied.  Then  by  an  irre- 
sistible deductive  swoop  he  reached  conclu- 
sions that  must  likewise  be  true.  In  such 
philosophy  there  is  little  need  of  verification. 

In  science,  and  therefore  also  in  the  reason- 
ing of  practical  life,  the  great  question  always 
is  whether  the  premises  are  true,  or  partly 
true,  or  false.  The  old  notion  was  that  deduc- 
tion led  to  certainty,  and  induction  did  not. 
But  in  the  scientific  method  the  object  is  not 
merely  to  deduce  consequences  from  laws  or 
principles,  but  to  establish  the  truth  or  falsity 
of  those  laws  or  principles  themselves.  Hence 
there  is  an  incessant  interplay  of  induction 
and  deduction.  Darwin's  distrust  of  deductive 
reasoning  was  due  to  his  fear  of  the  premises. 


DARWIN'S    VIEWS  OF  METHOD.  39 

But  he  said  that  all  his  hypotheses  had  to 
be  abandoned  or  modified,  and  they  were  the 
conclusions  of  inductions;  so  that  inductive 
reasoning  by  itself  is  also  absolutely  worth- 
less. The  truth  is,  Darwin  trusted  nothing. 
Induction  furnished  him  hypotheses,  and  de- 
duction interpreted  known  facts  and  led  to  new 
ones  under  those  hypotheses;  but  verification 
of  his  deductions  was  as  indispensable  to  him 
as  sunshine  to  a  plant. 

Darwin  himself  said,  "Any  fool  can  gen- 
eralize and  speculate."  There  has  always  been 
an  over-abundance  of  reasoning,  both  inductive 
and  deductive.  Untrammeled  induction  is 
largely  responsible  for  the  wild  and  worthless 
beliefs  that  have  burdened  the  world;  and 
untrammeled  deduction  is  as  largely  respon- 
sible for  the  dogmatic  dry-rot  that  has  pre- 
vented progress  in  human  discovery  and  beliefs. 
Darwin  was  one  of  the  most  powerful  deductive 
reasoners  that  ever  lived;  and  he  was  perfectly 
fearless  in  making  inductions,  for,  as  he  said 
himself,  he  made  an  hypothesis  on  every  sub- 
ject. But  the  illustrations  of  the  various  logi- 
cal processes  that  have  been  drawn  from  his 
works  show  that  even  he  —  with  his  almost 
superhuman  powers  of  observation,  his  innate 
desire  to  refer  every  fact  to  a  general  law,  his 


40  THE  METHOD   OF  DARWIN. 

rare  ability  to  reason  out  the  consequences  of 
his  hypotheses,  and  his  unbending  determina- 
tion to  test  his  reasoning  by  ruthless  investiga- 
tion —  made  important  inductions,  deductions, 
and  analogies  that  were  not  true,  and  failed  to 
make  deductions  that  should  have  thrust  them- 
selves upon  him.  He  rarely,  however,  fell 
into  the  old  and  vicious  error  of  thinking  that 
reasoning  of  any  kind  is  final  proof. 

He  built  up  a  large  inductive  structure  in 
pangenesis  only  to  see  it  rejected;  he  went 
wrong  in  his  deduction  concerning  the  relation 
between  high  degree  of  specialization  and  the 
chances  in  favor  of  preservation  of  a  species; 
and  was  prevented  by  a  bad  analogy  from 
investigating  the  effects  of  cross-  and  self-fer- 
tilization in  plants  until  the  subject  was  thrust 
upon  him  by  empirical  observation.  He  was 
as  productive  of  hypotheses  as  Nature  is  of  liv- 
ing things,  and,  like  her,  he  subjected  them 
all  to  the  principle  of  natural  selection.  His 
mind  was  so  fertile  in  guesses  and  so  quick  in 
testing  them  that  he  called  much  of  his  work 
"fool's  experiments. "  But  in  this  way  nothing 
escaped  him. 

In  recent  years  there  has  been  made  a  for- 
mal statement  of  the  reasons  why  fertility  in 
hypotheses  should  be  cultivated,  and  how  they 


DARWIN'S    VIEWS  OF  METHOD.  41 

should  be  used.1  The  principle  of  multiple 
hypotheses  is  urged,  because  in  the  sciences 
it  is  not  generally  possible  to  hit  at  once  upon 
a  cause  from  a  study  of  the  facts.  As  many 
hypotheses  as  possible  should  be  invented  to 
explain  the  facts  under  investigation,  and  as 
fast  as  possible,  as  new  light  comes,  other 
hypotheses  should  be  added,  in  order  that  the 
mind  may,  as  far  as  possible,  put  itself  in 
possession  of  all  the  possible  causes  of  the 
phenomena.  By  keeping  them  all  constantly 
before  it,  it  can  consider  every  fact  from  many 
points  of  view;  and  each  hypothesis  will  fur- 
nish its  own  clews  to  further  evidence.  In 
this  way,  also,  the  mind  can  more  easily  keep 
itself  in  a  judicial  attitude.  With  the  increase 
of  knowledge  some  of  the  hypotheses  are  shown 
to  be  inadequate,  and  by  the  process  of  exclu- 
sion their  number  is  reduced  until  the  inves- 
tigation ends  in  the  establishment  of  the  true 
theory  of  the  cause  of  the  facts  under  inves- 
tigation. 

In  actual  practice  a  good  many  difficulties 
are  connected  with  this  method  of  using 
hypotheses.  Among  a  number  of  hypotheses 
one  will  almost  invariably  have  a  somewhat 
higher  degree  of  probability  than  the  rest;  the 

1  T.  C.  Chamberlain,  in  Science,  Feb.  7,  1890. 


42  THE  METHOD   OF  DARWIN. 

mind,  working  along  the  lines  of  least  resist- 
ance, follows  the  clews  it  offers  to  the  neglect 
of  the  other  hypotheses.  The  more  usual  prac- 
tice is  typified  in  Darwin's  method.  He  made 
an  hypothesis  at  the  earliest  possible  moment, 
and  began  to  work  with  it.  With  increasing 
knowledge  it  was  modified,  or  rejected  and 
replaced  by  a  more  likely  one.  So  that  there 
was  a  succession  of  hypotheses  or  of  improve- 
ments of  the  original  one. 

The  method  of  multiple  hypotheses  is  com- 
mon enough,  if  it  be  made  to  include  not  only 
the  instances  in  which  the  same  individual 
entertains  several  hypotheses,  but  also  those 
in  which  different  hypotheses  are  entertained 
by  different  people.  There  are  few  questions 
on  which  there  are  not  several  opinions;  and 
one  approaching  the  subject  impartially  con- 
siders them  together  in  order  to  adopt  the  most 
likely  one.  The  process  of  exclusion  works 
admirably,  and  the  result  amounts  to  demon- 
stration when  all  the  possible  hypotheses  are 
known,  and  one  needs  only  to  show  that  one 
of  them  agrees  with  the  facts  and  the  others 
do  not.  Newton,  in  establishing  the  law  of 
gravitation,  showed  that  the  orbits  and  veloci- 
ties of  the  planets  would  be  what  they  are  if 
the  attractive  force  resided  at  the  centre  of  the 


DARWIN'S   VIEWS  OF  METHOD.  43 

system  or  in  the  sun,  and  acted  with  a  force 
varying  inversely  as  the  square  of  the  dis- 
tance; and  that  they  could  not  be  what  they 
are  if  the  force  were  located  anywhere  except 
at  the  centre.  In  this  instance  Newton  was 
able  to  exclude  mathematically  every  theory 
except  the  true  one;  and  the  demonstration 
was  made  complete  by  the  positive  proof  that 
the  facts  accorded  with  the  one  hypothesis  and 
were  at  variance  with  every  other. 

Darwin  used  the  principle  of  exclusion  in 
one  of  his  early  scientific  efforts.  There  were 
two  hypotheses  to  account  for  the  existence  of 
the  great  terraces  called  benches,  or  parallel 
roads,  of  Glen  Roy,  in  Scotland.  It  was  evi- 
dent enough  that  they  had  been  formed  by  the 
action  of  water;  and  they  must  either  have 
been  formed  by  the  sea,  in  which  case  the 
uppermost,  for  example,  must  have  been  after- 
wards elevated  more  than  one  thousand  feet; 
or  they  must  have  been  the  ancient  shores  of 
a  lake  formed  by  the  blocking  up  of  the 
open  side  of  the  now  empty  lake  bed.  Darwin 
studied  the  region,  and  concluded  that  the 
benches  were  of  marine  origin.  He  could  not 
conceive  them  to  be  due  to  barriers  of  rock  or 
detritus.  He  was  then  fresh  from  his  studies 
on  the  geology  of  South  America.  On  the 


44  THE  METHOD   OF  DARWIN. 

coasts  of  that  continent  he  had  grown  familiar 
with  the  immense  scale  on  which  elevations 
take  place,  and  had  many  opportunities  to 
study  marine  sand  and  gravel  formations  that 
are  now  hundreds  of  feet  above  sea  level.  It 
was  therefore  as  easy  for  him  to  conceive  a 
marine  origin  of  the  terraces  of  Glen  Roy  as 
it  was  difficult  for  him  to  believe  that  they 
were  the  old  shores  of  an  elevated  lake  that 
had  been  blocked  in.  He  adopted  the  method 
of  exclusion,  proved  the  one  hypothesis  prob- 
able and  the  other  improbable. 

But  this  instance  of  the  principle  of  exclu- 
sion illustrates  well  the  danger  to  which  the 
investigator  is  exposed  in  its  use.  In  geology, 
zoology,  etc., —  in  what  Darwin  calls  the  mixed 
sciences,  —  one  can  rarely  know  whether  all 
the  possible  hypotheses  are  known.  Newton 
could  make  a  rigid  demonstration,  not  simply 
because  he  could  treat  his  problem  mathe- 
matically, but  because  he  was  able  to  treat 
all  the  possible  hypotheses.  When  the  gla- 
cial theory  was  introduced  into  the  geological 
world  the  ancient  terraces  high  above  sea 
level  in  the  colder  temperate  zone  were  ac- 
counted for.  Then  it  was  plain  that  the  ter- 
races of  Glen  Roy  had  been  the  shores,  not 
of  the  sea,  nor  of  a  lake  dammed  up  by  rock 


DARWIN'S    VIEWS   OF  METHOD.  45 

or  detritus,  but  of  a  lake  dammed  up  by  gla- 
cial ice. 

Darwin  was  ashamed  of  his  arguments  and 
conclusions  about  Glen  Roy.  "My  error,"  he 
said,  "has  been  a  good  lesson  to  me  never  to 
trust  in  science  to  the  principle  of  exclusion."  l 
But  it  is  inevitable  that  apparently  definite 
views  should  receive  just  such  shocks  upon 
the  introduction  of  a  great  new  principle  in  a 
science.  The  facts  that  find  their  explanation 
under  the  single  newly  discovered  cause  are 
necessarily  referred,  before  the  advent  of  the 
new  hypothesis,  to  very  various  and  unrelated 
causes. 

On  the  question  of  the  origin  of  species 
there  were  really  two  hypotheses,  creation  and 
descent,  when  Darwin  took  hold  of  it;  and  he 
adopted  the  process  of  exclusion  in  treating 
them.  The  evidence  was  all  in  favor  of  descent 
by  natural  selection  and  opposed  to  creation. 
But  he  was  himself  emphatic  in  the  declaration 
that  the  origin  of  species  by  natural  selec- 
tion was  not  demonstrated.  Belief  in  it  must 
be  based  on  general  considerations,  —  that 
natural  selection  is  an  actually  existing  cause, 
and  that  it  explains  a  host  of  facts  and  brings 
them  under  one  point  of  view.  One  hypothesis 

1  Life  and  Letters,  Vol.  I.  p.  57. 


46  THE  METHOD   OF  DARWIN. 

was  excluded  and  the  other  adopted.  And  the 
one  that  was  accepted  was  based,  not  on  direct 
proof,  but  on  one  of  the  most  magnificent  series 
of  deductions  that  the  world  has  ever  seen. 

The  further  discussion  of  this  subject  will 
be  deferred  for  the  present.  In  the  following 
chapters  Darwin's  method  of  treating  the  prob- 
lems that  presented  themselves  to  his  mind 
will  be  analyzed  in  some  detail,  and  the  clos- 
ing chapter  will  deal  with  the  logical  history 
of  the  principle  of  natural  selection. 


III. 


STARTING  POINTS.  — METHOD  OF  STARTING  IN- 
VESTIGATIONS.—  ISOLATED    PHENOMENA. 

THE  starting  points  of  many  of  Darwin's 
researches  were  furnished  him  by  other 
intelligent  men.  In  many  cases  these  men  not 
only  were  in  possession  of  the  facts,  but  had 
hit  upon  their  true  explanation.  With  the 
facts  thrust  upon  them,  with  enough  reasoning 
ability  to  pursue  them,  they  gave  away  their 
heritage,  —  luckily  to  one  who  knew  its  value. 
In  his  frank  but  modest  analysis  of  his  own 
mental  qualities  he  said  of  himself,  as  already 
quoted,  "  I  think  I  am  superior  to  the  com- 
mon run  of  men  in  noticing  things  which 
easily  escape  attention,  and  in  observing  them 
carefully.  .  .  .  From  my  earliest  youth  I  have 
had  the  strongest  desire  to  understand  or  ex- 
plain whatever  I  observed,  .  .  .  that  is,  to  group 
all  facts  under  some  general  laws."1  There 
can  be  no  doubt  that  his  great  interest  in 
apparently  little  things,  and  his  efforts  to  make 

1  Life  and  Letters,  Vol.  I.  p.  83. 


48  THE  METHOD   OF  DARWIN. 

the  most  of  them,  were  due  to  his  conviction 
that  important  things  were  hidden  behind  them, 
that  they  were  illustrations  of  general  laws. 

Lawson,  Vice-Governor  of  the  Galapagos 
Islands  at  the  time  of  Darwin's  visit,  knew 
that  the  tortoises  of  the  different  islands  dif- 
fered from  one  another,  and  even  declared  to 
Darwin  that  he  could  tell  from  which  island 
any  tortoise  came.1  He  had  the  time,  and 
the  material  lay  at  his  feet ;  but  he  left  it  for 
Darwin  to  make  the  Galapagos  Islands  famous 
as  illustrations  of  the  local  variations  of 
species.  Darwin  himself  had  to  have  the  evi- 
dence thrust  upon  him  from  several  directions 
before  he  grasped  its  significance,  but  his 
greater  appreciation  of  the  nature  and  value 
of  the  facts  made  him  their  master. 

After  his  return  from  the  Beagle  voyage, 
Mr.  Wedgwood  of  Maer  Hall  suggested  to 
him  that  the  apparent  sinking  of  superficial 
bodies,  ashes,  marl,  cinders,  etc.,  into  the 
earth  is  due  to  the  action  of  earth-worms.2 
Both  the  facts  and  the  theory  were  ready  at 
hand.  To  the  one  man  they  were  probably 
interesting  as  intellectual  playthings;  to  the 
other  they  became  the  starting  point  for  a 

1  Naturalist's  Voyage  around  the  World,  pp.  393-398. 

2  The  Formation  of  Vegetable  Mould,  etc.,  p.  3. 


STARTING  POINTS.  49 

long  investigation.  Darwin  read  one  of  his 
first  papers,  "  On  the  Formation  of  Vegetable 
Mould,"  before  the  Geological  Society  of  Lon- 
don, November  1,  1837;  and  on  the  same  ap- 
parently insignificant  subject  he  published  the 
last  book  of  his  life. 

While  collecting  in  the  Chonos  Archipelago, 
Southern  Chile,  he  found  numbers  of  an  insig- 
nificant little  cirriped,  none  more  than  one 
tenth  of  an  inch  long,  embedded  in  the  shell  of 
Concholepas  peruviana.1  The  zoological  mate- 
rial of  his  trip  was  turned  over  to  specialists 
for  description,  he  furnishing  the  field  notes 
and  editing  the  publications.  Much  zoological 
information  was  thus  given  to  the  world,  but 
none  of  all  that  material  ever  served  as  a  start- 
ing point  for  a  great  investigation.  The  little 
abnormal  cirriped  was  left  to  Darwin  himself; 
probably  because  it  was  too  small  an  affair  to 
be  taken  charge  of  by  others.  In  his  hands  it 
became  the  germ  of  a  monograph  on  the  Cirri- 
pedia,  which  is  still  the  classical  literature 
of  the  group.  To  determine  its  position  he 
studied  the  structure  of  as  many  genera  as 
possible.  Dr.  J.  E.  Gray,  who  had  already 
collected  a  large  amount  of  material  for  a  mon- 

1  A   Monograph  of  the   Cirripedia,  Vol.  I.,  Preface,  p.  5 ; 
Vol.  II.  pp.  566-586. 

4 


$O  THE  METHOD   OF  DARWIN, 

ograph  of  the  group,  turned  it  over  to  Darwin. 
Gray  did  many  things,  but  none  well  enough 
to  make  what  he  wrote  indispensable  in  the 
study  of  any  subject;  he  will  be  remembered 
chiefly  as  a  keeper  of  the  zoological  depart- 
ment of  the  British  Museum,  and  as  a  bitter 
opponent  of  Darwin's  theory  of  descent,  while 
the  latter' s  monograph  heads  the  list  of  cirri- 
ped  literature. 

Boitard  and  Corbie"  merely  made  the  obser- 
vation that,  when  they  crossed  certain  breeds 
of  pigeons,  birds  colored  like  the  Columba 
livia,  or  common  dove-cot,  were  almost  inva- 
riably produced.1  It  drew  Darwin's  attention 
and  led  to  numerous  experiments  on  rever- 
sion due  to  crossing.  Certainly  some,  perhaps 
many,  scientific  men  had  known  that  the  species 
of  sundew  (Droserd)  catch  insects.  Darwin 
himself  had  heard  as  much.  Exhausted  by 
his  long  labors  on  the  Origin  of  Species,  he 
was  resting  near  Hartfield  during  the  summer 
.of  1860,  and  "was  surprised  by  finding  how 
large  a  number  of  insects  were  caught  by  the 
leaves  of  the  common  sundew  (Drosera  rotundi- 
folid)  on  a  heath  in  Sussex."  2  The  right  mind 

1  The  Variation  of  Animals  and  Plants  under  Domestica- 
tion, Vol.  II.  p.  14. 

2  Life  and  Letters,  Vol.  I.  p.  77. 


STARTING  POINTS.  51 

had  been  impressed,  and  away  the  giant  intel- 
lect started  on  another  long  and  weary,  but  suc- 
cessful, search  after  truth.  Mr.  W.  Marshall 
knew  that  in  the  mountains  of  Cumberland 
many  insects  adhered  to  the  leaves  of  Pin- 
guicula;  he  told  Darwin,  and  Darwin  told  the 
world.1  Mr.  Holland's  statement  that  water 
insects  are  often  found  imprisoned  in  the 
bladders  of  Utricularia  is  interesting,  chiefly 
because  it  led  Darwin  to  investigate  the 
genus.2 

The  most  fertile  suggestions,  however,  came 
to  him  from  the  facts  brought  out  by  his  own 
work.  He  did  not  record  the  hundredth  part 
of  the  tentative  notions  that  entered  his  mind; 
but  many  of  the  most  important  and  lasting 
had  their  rise  in  what  to  most  other  men  is  the 
refuse  heap  of  curious  and  exceptional  little 
facts.  Perhaps  one  of  the  noblest  lessons  he 
left  to  the  world  is  this,  —  which  to  him 
amounted  to  a  profound,  almost  religious  con- 
viction, —  that  every  fact  in  nature,  no  matter 
how  insignificant,  every  stripe  of  color,  every 
tint  of  flowers,  the  length  of  an  orchid's  nec- 
tary, unusual  height  in  a  plant,  all  the  infinite 
variety  of  apparently  insignificant  things,  is 

1  Insectivorous  Plants,  p.  369. 

2  Ibid.,  p.  395. 


52  THE  METHOD   OF  DARWIN. 

full  of  significance.  For  him  it  was  a  histor- 
ical record,  the  revelation  of  a  cause,  the  lurk- 
ing place  of  a  principle. 

A  typical  example  of  his  treatment  of  little 
exceptional  facts  is  that  of  "Hero, "the  un- 
usual plant  in  one  of  the  later  generations  of 
self -fertilized  plants  of  Ipomoea  purpurea.^  It 
was  a  little  -larger  than  the  crossed  seedlings 
of  the  same  generation,  and  the  first  exception 
that  had  arisen,  in  his  many  experiments,  to 
the  rule  that  the  crossed  are  superior  to  the 
self-fertilized  seedlings  in  size  and  vigor.  It 
was  a  little  thing,  even  for  an  exception,  and 
had  occurred  only  after  a  very  long  series  had 
established  the  rule.  It  was  very  fit  to  suffer 
the  common  fate  of  exceptions,  and  to  be 
deliberately  choked  for  the  benefit  of  the  rule 
to  which  it  was  so  inconveniently  related.  By 
other  hands  it  would  probably  have  been  re- 
corded and  then  ignored.  Darwin  made  it  the 
parent  of  a  whole  race  of  exceptions.  He 
found  "Hero"  to  be  exceptional,  not  only  in 
being  unusually  tall,  but  in  its  being  highly 
self -fertile,  in  its  great  powers  of  growth,  and  in 
its  descendants  which  were  crossed  having  no 
advantage  over  its  self-fertilized  descendants. 

His  aim,  as  soon  as  he  hit  upon  a  line  of 
1  Effects  of  Cross-  and  Self-Fertilization,  p.  60. 


METHOD  OF  STARTING  INVESTIGATIONS.    53 

investigation,  was  to  reach  as  quickly  as  pos- 
sible a  crucial  test  or  a  crucial  observation 
that  would  enable  him  to  determine  positively 
whether  or  not  his  beliefs  were  justified.  As 
soon  as  the  idea  of  descent  of  species  took 
definite  shape  in  his  mind,  he  determined, 
after  deliberation,  to  take  up  the  study  of 
domestic  pigeons.1  He  selected  these  because 
the  variations  were  more  numerous  and  plainer, 
more  of  them  had  arisen  in  the  historical  period 
than  is  usual  with  animal  groups,  the  material 
was  abundant  and  easily  accessible,  etc.  He 
cnose  for  his  investigation  the  conditions  most 
favorable  to  success. 

When  he  discovered  that  insects  are  caught 
in  large  numbers  by  the  common  sundew,  he 
gathered  a  number  of  plants,  counted  the  leaves 
and  the  number  that  had  caught  insects ;  and 
compared  the  results  with  the  accidental  de- 
struction of  insects  by  the  viscous  buds  of  the 
horse-chestnut,  etc.2  Similar  results  were  pro- 
duced by  somewhat  similar  means;  but  by  the 
comparison  he  secured  preliminary  evidence 
that  the  leaves  of  the  sundew,  unlike  the  buds 
of  the  horse-chestnut,  were  excellently  adapted 

1  Origin   of   Species,   p.    15;    Variation    of    Animals   and 
Plants  under  Domestication,  Vol.  I.  p.  137. 

2  Insectivorous  Plants,  pp.  1-3,  63-76. 


54  THE  METHOD   OF  DARWIN. 

for  catching  insects.  To  him  adaptation  for 
insect  catching  meant  that  this  habit  was 
advantageous  to  the  plant;  and  that  it  must 
derive  nourishment  from  the  captured  insects. 
He  knew  what  elements  plants  required  for 
nourishment,  and  immediately  set  about  mak- 
ing another  crucial  test.  When  led  to  believe 
that  the  leaves  absorbed  nutritious  matter  from 
the  insects,  he  made  a  crucial  experiment  by 
immersing  numbers  of  the  leaves  in  nitro- 
genous and  non-nitrogenous  fluidsW  the  same 
specific  gravity  to  find  whether  they  would 
act  differently  in  the  two  cases,  taking  care 
that  the  two  sets  of  conditions  should  be  the 
same  except  in  the  presence  of  nitrogenous 
matter  in  the  one  and  its  absence  in  the  other. 
The  test  confirmed  his  belief,  and  is  an 
example  of  the  most  rigid  type  of  reasoning 
and  experiment,  —  a  combination  of  positive 
and  negative  evidence  which  Mill  called  the 
Method  of  Difference.  Darwin's  clear  notion 
of  what  constitutes  good  evidence  led  him  to 
seek  demonstrative  evidence  by  the  shortest 
way.  In  the  case  just  described  he  secured  it 
at  once,  and  might  have  rested  content  with 
having  established  the  principle;  but  as  soon 
as  he  found  that  the  nitrogenous  fluid  alone 
excited  energetic  movements  "  it  was  obvious 


ME  THOD  OF  STAR  TING  INVES  TIG  A  TIONS.    5  5 

that  here  was  a  fine  new  field  for  investiga- 
tion." His  crucial  tests  only  gave  him  confi- 
dence that  there  was  more  beyond;  then  he 
began  the  long  series  of  observations  and 
experiments  which  resulted  in  the  charming 
volume  on  "Insectivorous  Plants." 

It  is  told  elsewhere  how  he  was  deterred  by 
theoretical  considerations  from  experimenting 
on  the  effects  of  cross-  and  self-fertilization, 
and  how  the  expectation  of  early  results  was 
fairly  thrust  upon  him  by  the  difference  in  size 
and  vigor  between  crossed  and  self-fertilized 
seedlings.1 

When  once  his  attention  was  fixed,  he  made 
a  preliminary  experiment  on  two  plants,  with 
the  effects  of  cross-  and  self-fertilization  as  the 
principal  object  of  investigation.  The  results 
corroborated  his  previous  observations,  and 
he  was  in  possession  of  the  principle.  Such 
simple  preliminary  experiments  are  interest- 
ing, since,  if  they  do  not  establish  a  principle 
fully,  they  raise  up  for  it  a  higher  degree  of 
probability  than  any  succeeding  experiments, 
and  make  it  possible  to  work  deductively  with 
considerable  confidence. 

The  introduction  of  the  principle  of  con- 
tinuity into  general  scientific  thinking  has 

1  Effects  of  Cross-  and  Self-Fertilization,  pp.  6-8. 


56  THE  METHOD   OF  DARWIN. 

made  it  a  normal  intellectual  process  to  look 
upon  exceptional  and  isolated  phenomena  as 
merely  extreme  instances  of  much  larger 
groups.  One  of  the  marked  characteristics  of 
Darwin's  work  is  that  he  selected  such  extreme 
instances,  and  sought  to  connect  them  with  the 
more  common  facts  to  which  they  were  related, 
by  proving,  or  at  least  suggesting,  their  deri- 
vation from  the  latter.  Wherever  it  was  pos- 
sible in  his  experiments,  he  varied  the  amount 
of  a  cause  in  order  to  note  the  proportionate 
variation  in  the  amount  of  the  effect;  and 
where  he  had  to  depend  upon  observation  alone, 
he  made  strenuous  efforts  to  connect  extreme 
instances  by  gradations  of  character. 

Thus,  he  and  his  son  Francis,  by  continuous 
attention  to  the  sleeping  movements  of  plants, 
were  able  to  show  that  it  is  not  true,  as  is  gen- 
erally supposed,  that  the  leaves  move  only  in  the 
evening  when  going  to  sleep,  and  in  the  morn- 
ing when  awaking ;  for  they  found  no  exception 
to  the  rule  that  leaves  which  sleep  continue  to 
move  during  the  whole  twenty-four  hours,  only 
moving  more  quickly  when  going  to  sleep  and 
awaking  than  at  other  times.1  They  were  able 
to  show  that  sleeping  movements  are  only 
liighly  specialized  and  exaggerated  modifica- 

1  Power  of  Movement  in  Plants,  p.  403. 


ISOLATED  PHENOMENA.  57 

tions  of  the  universal  movement  of  circumnu- 
tation.  In  his  experiments  on  insectivorous 
plants  with  phosphate  of  ammonia  he  varied 
the  proportion  of  the  latter  to  determine  how 
small  an  amount  would  affect  the  tentacles  of 
Drosera.1  He  found  that  excessively  minute 
quantities  of  the  latter  would  produce  reac- 
tion. His  results  were  so  astonishing  that  in 
1873  he  doubted  his  own  experiments  of  1872, 
and  in  1874  he  again  thought  that  some  mis- 
take must  have  been  made,  and  again  repeated 
the  experiments,  but  always  with  the  same 
results.  He  discussed  these  remarkable  facts 
at  some  length,  tried  to  make  them  more  cred- 
ible by  comparing  them  with  similar  cases  that 
are  equally  astonishing  but  are  known  to  be 
true;  and  expressed  the  hope  that  his  experi- 
ments would  be  repeated,  at  the  same  time 
laying  down  the  conditions  of  success. 

While  studying  the  power  of  circumnutation 
in  plants  the  Darwins  accidentally  left  some 
of  their  specimens  in  several  cases  exposed  to 
oblique  light.  Before  they  "knew  how  greatly 
ordinary  circumnutation  was  modified  by  a 
lateral  light,  some  seedling  oats,  with  rather 
old  and  therefore  not  highly  sensitive  cotyle- 
dons, were  placed  in  front  of  a  northwest  win- 

1  Insectivorous  Plants,  pp.  154-173., 


58  THE  METHOD   OF  DARWIN. 

dow,  towards  which  they  bent  all  day  in  a 
strongly  zigzag  course.  On  the  following  day 
they  continued  to  bend  in  the  same  direction, 
but  zigzagged  much  less.  The  sky,  however, 
became,  between  12  140  and  2  135  P.M.,  overcast 
with  extraordinarily  dark  thunder-clouds,  and 
it  was  interesting  to  note  how  plainly  the 
cotyledons  circumnutated  during  this  inter- 
val."1 These  observations  they  considered  of 
some  value  from  their  having  made  them  while 
they  were  not  attending  to  heliotropism ;  and 
they  were  led  by  them  "to  experiment  on 
several  kinds  of  seedlings,  by  exposing  them 
to  a  dim  lateral  light,  so  as  to  observe  the  gra- 
dations between  ordinary  circumnutation  and 
heliotropism."  An  accidental  observation  led 
to  variations  in  the  experiments,  which  re- 
sulted in  demonstrating  continuity  between  two 
apparently  distinct  classes  of  movements. 

In  some  of  his  remarkable  studies  on  grada- 
tions of  characters,  where  it  was  impossible  to 
make  experiments,  he  sought  out  and  observed 
Nature's  own  variations.  Perhaps  the  most 
striking  instance  of  the  study  of  gradations  of 
character  is  that  connected  with  the  "ocellus  " 
on  the  tail  coverts  of  the  peacock.2  This 

1  Power  of  Movement  in  Plants,  p.  421. 

2  Descent  of  Man,  Vol.  II.  pp.  132-145 


ISOLATED  PHENOMENA.  59 

feather-mark  was  properly  considered  a  serious 
difficulty  to  Darwin's  theory  because  of  its 
remarkable  character.  But  with  consummate 
ingenuity  he  undertook  to  connect  it  by  a 
series  of  less  and  less  remarkable  markings 
with  the  ordinary  feather-markings  of  the 
group  to  which  the  peacock  belongs.  It  is 
impossible  to  exaggerate  the  importance  of 
studying  phenomena  in  their  quantitative  and 
qualitative  variations,  for  on  it  depends  the 
establishment  of  continuity  between  phenom- 
ena apparently  widely  separated,  and  it  fre- 
quently leads  to  results  that  can  be  reached  in 
no  other  way. 


IV. 


EXHAUSTIVENESS.  — TIME  GIVEN  TO  INVESTI- 
GATIONS.—TREATMENT  OF  OBJECTIONS. 

THE  theories  with  which  Darwin  dealt  were 
so  general,  and  the  facts  that  had  to  be 
handled  as  evidence  were  so  vast  in  number, 
that  probably  no  man  was  ever  exposed  to 
greater  temptation  to  collect  his  evidence  pro- 
miscuously from  all  quarters,  picking  up  in 
each  field  what  was  already  known,  and  sup- 
plementing it  by  a  few  test  observations.  But 
he  never  contented  himself  with  sketching 
theories  and  adorning  them  with  dashes  of 
evidence. 

He  made  himself  invincible  by  the  exhaust- 
iveness  with  which  he  determined  the  quality 
of  his  evidence.     The  great  confidence  which  \ 
scientific  men  have  had  in  him  has  been  due  | 
to  the  fact  that  he  did  not  leave  it  to  them  to  . , 
test  the  theories  which  he  presented.      He  con- 
vinced  the  world    of   the  truth  of  a  doctrine 
which  others  had  striven  in  vain  for  fifty  years 
to  establish.     To  my  mind  one  of  the  chief 


EXHA  US  TIVENESS.  6 1 

characteristic  differences  between  his  work 
and  that  of  Lamarck  and  others,  apart  from 
differences  in  the  explanations  offered,  is  its 
superb  exhaustiveness.  It  is  as  impossible 
now  to  take  the  ideas  of  descent  and  of  natural 
selection  out  of  the  world  as  to  take  a  star  out 
of  the  sky.  The  firm  establishment  of  these 
ideas  was  due  to  the  quality  and  quantity  of 
Darwin's  work,  and  both  of  these  were  deter- 
mined by  the  same  exhaustiveness  in  method. 

When  he  started  out  to  describe  the  single 
little  abnormal  cirriped  from  the  west  coast 
of  South  America,  he  was  characteristically 
led,  as  he  said,  for  the  sake  of  comparison,  to 
examine  the  internal  parts  of  as  many  genera 
as  he  could  procure.  This  untamed  determina- 
tion to  find  out  all  there  was  to  know  about 
what  he  was  describing  was  associated  with  a 
fine  contempt  for  the  kind  of  work  that  merely 
describes  new  things  without  showing  all  their 
connections.  One  of  the  greatest  and  most 
constant  obstacles  to  his  progress  was  that  this 
intellectual  quality  was  so  rare  or  so  little  cul- 
tivated in  other  naturalists;  so  much  of  the 
scientific  material  with  which  he  had  to  deal 
was  so  superficially  or  carelessly  worked  out 
that  he  never  knew  what  to  trust. 

Among  the  best  examples  of  this  spirit  of 


62  THE  METHOD   OF  DARWIN. 

exhaustiveness  is  his  study  of  pigeons.1  As 
usual,  he  knew  clearly  what  he  was  after,  and 
this  gave  him  the  power  of  selecting  judiciously 
the  lines  along  which  to  make  investigations 
and  of  using  to  the  best  advantage  the  materials 
he  worked  on.  In  his  remarks  on  the  search 
for  the  cause  of  the  modification  of  species,  he 
said,  "  Believing  that  it  is  always  best  to  study 
some  special  group,  I  have,  after  deliberation, 
taken  up  domestic  pigeons."  With  other  ends 
in  view,  pigeons  might  be  studied  in  different 
ways.  But  remembering  his  purpose,  his  work 
on  pigeons  is  a  model  of  exhaustiveness  as  well 
as  of  reasoning.  He  not  only  studied  the  vari- 
ation of  breeds,  but  sought  its  explanation  by 
a  minute  study  of  individual  differences.  He 
considered  the  skeleton  as  well  as  the  feathers, 
and  gathered  facts  and  specimens  from  all  over 
the  world. 

What  is  true  of  his  study  of  pigeons  is  true 
of  his  work  on  orchids.  The  adaptation  of 
flowers  for  cross-fertilization  had  interested  him 
for  many  years,  and  he  had  collected  a  large 
mass  of  observations;  but  he  was  true  to  his 
instinct :  "  It  seemed  to  me  a  better  plan  to 
work  out  one  group  of  plants  as  carefully  as  I 

1  Variation  of  Animals  and  Plants  under  Domestication, 
Chaps.  V.  and  VI.,  pp.  137-235. 


EXHAUSTIVE  NESS.  63 

could  rather  than  publish  many  miscellaneous 
and  imperfect  observations."  Orchids  furnished 
an  extreme  case ;  and  his  work  on  them  is  fas- 
cinating from  the  nature  of  the  subject,  the  end 
aimed  at,  and  the  ingenuity  of  the  reasoning 
employed.  He  showed  "how  admirably  these 
plants  are  constructed  so  as  to  permit  of,  or  to 
favor,  or  to  necessitate  cross-fertilization  " ;  but 
the  way  in  which  he  did  it  is  as  admirable  from 
a  logical  point  of  view  as  the  flowers  them- 
selves are  in  their  peculiar  adaptation.  By 
thus  selecting  judiciously  the  most  extreme 
special  cases  for  exhaustive  examination,  he 
threw  the  strongest  light  on  all  the  collateral 
evidence,  and  made  it  easy  for  him  to  under 
stand  its  significance.  On  the  shoulders  of 
such  work  his  theories  sat  firmly,  and  it  made 
it  easy  for  those  who  came  after  him  to  work 
out  the  classes  of  facts  which  he  was  not  able 
to  exhaust. 

In  each  of  the  many  special  studies  which  he 
carried  on  there  are  many  models  of  method  in 
the  pursuit  of  details.  The  following  case  is 
especially  interesting  because  it  illustrates  both 
the  habitual  care  of  the  authors  in  their  experi- 
ments on  the  movements  of  plants  and  the  ex- 
treme liability  to  error  that  results  from  a 
wrong  start.  Since  the  book  on  the  "  Power  of 


64  THE  METHOD   OF  DARWIN. 

Movement  in  Plants  "  was  written,  it  has  been 
shown  that  the  conclusion  of  the  authors  that 
"an  object  which  yields  with  the  greatest  ease 
will  deflect  a  radicle  "  is  wrong.  The  whole 
superstructure  of  reasoning  which  resulted  in 
the  notion  that  the  tips  of  radicles  are  sensitive 
to  contact  was  therefore  built  on  sand.  The 
experiments  and  reasoning  will  be  discussed 
from  the  point  of  view  of  the  authors  at  the 
time  they  were  made,  and  afterwards  attention 
will  be  called  to  the  corrections  that  have  since 
been  made  by  others.  In  their  work  on  the 
movements  of  radicles,  Charles  and  Francis 
Darwin  found  that  "an  object  which  yields 
with  the  greatest  ease  will  deflect  a  radicle." 
Extremely  thin  tin-foil  on  soft  sand  was  not  at 
all  impressed,  and  deflected  the  root  at  right 
angles.  Hence,  they  reasoned,  the  cause  "of 
the  deflection  could  not  be  mechanical  contact. 
A  conceivable  hypothesis  was  that "  the  gentlest 
pressure  might  arrest  growth  and  the  apex  grow 
only  on  one  side;  but  this  view  leaves  unex- 
plained the  curvature  of  the  upper  part,  extend- 
ing for  a  length  of  8-10  mm.  .  .  .  We  were 
therefore  led  to  suspect  that  the  apex  was  sen- 
sitive to  contact,  and  that  an  effect  was  trans- 
mitted from  it  to  the  upper  part."1  By  the 

1  Power  of  Movement  in  Plants,  pp.  131-140. 


EXHAUSTIVE  NESS.  65 

exclusion  of  the  other  two  hypotheses  —  me- 
chanical contact  and  arrest  of  growth  —  they 
confined  themselves  to  the  last  one,  sensitive- 
ness to  contact.  This  would  have  been  a  fine 
field  for  a  discussion  of  the  known  facts  fol- 
lowed by  a  necessary  inference.  Of  the  three 
possible  hypotheses,  two  had  been  excluded, 
and  the  third  must  be  true.  It  would  seem 
quite  clear  that  the  case  was  logically  proved. 
But  instead  of  making  this  the  end,  they  made 
it  the  beginning  of  their  work. 

They  "thought  that  any  small  hard  object 
affixed  to  the  tip  of  a  radicle  freely  suspended 
and  growing  in  damp  air,  might  cause  it  to 
bend  if  it  were  sensitive,  and  yet  would  not 
offer  any  mechanical  resistance  to  its  growth." 
The  results  of  their  experiments  proved  remark- 
able. When  approaching  the  subject  they  made 
a  preliminary  trial  with  seven  beans  at  a  rather 
cool  temperature,  and  six  radicles  curved.  To 
quote  again,  "These  six  striking  cases  almost 
convinced  us  that  the  apex  was  sensitive,  but 
of  course  we  determined  to  make  many  more 
trials."  As  they  had  noticed  that  radicles  grew 
much  more  quickly  when  subjected  to  con- 
siderable heat,  and  as  they  imagined  that  heat 
would  increase  their  sensitiveness,  they  made 
five  or  six  dozen  trials  on  more  than  two  dozen 
5 


66  THE  METHOD   OF  DARWIN. 

beans  at  a  temperature  of  69°-72°  F.  The 
result  was  moderately  distinct  deflection  in  only 
one  radicle;  in  five  other  cases  slight  and 
doubtful  deflection.  "We  were  astonished  at 
this  result,  and  concluded  that  we  had  made 
some  inexplicable  mistake  in  the  first  six  experi- 
ments. But  before  finally  relinquishing  the 
subject,  we  resolved  to  make  one  other  trial, 
for  it  occurred  to  us  that  sensitiveness  is  easily 
affected  by  external  conditions,  and  that  radi- 
cles growing  naturally  in  the  earth  in  the  early 
spring  would  not  be  subjected  to  a  temperature 
nearly  so  high  as  70°  F."  In  the  vast  num- 
ber of  successful  trials  that  they  made  they 
allowed  the  radicles  to  grow  at  a  temperature 
of  55°-6o°  F.1 

Had  they  stopped  with  the  first  trial,  they 
would  have  hit  the  explanation  which  they 
finally  adopted,  and  missed  the  effect  of  varia- 
tions in  temperature.  Had  they  stopped  with 
the  second,  the  question  would  have  hung  in 
the  balance  between  contradictory  results.  It 
was  very  feasible  to  reason  that  the  results  of 
the  older  experiment  were  due  to  some  error  of 
observation  or  manipulation.  Nothing  would 
have  been  known  concerning  the  effect  of  tem- 
perature, and  nothing  concerning  the  original 

1  Power  of  Movement  in  Plants,  pp.  141,  142. 


EXHAUSTIVENESS.  6/ 

question.  Their  imagination  had  led  them  to 
introduce  a  new  element  into  the  second  experi- 
ment; then  reason  prevented  them  from  suc- 
cumbing before  the  disturbance  in  the  results, 
and  led  them  to  recognize  it  as  a  determining 
factor  and  treat  it  as  such  in  their  subsequent 
experiments. 

The  contradictory  state  in  which  things  would 
have  been  left  at  the  end  of  their  second  experi- 
ment is  neatly  illustrated  by  another  case  in 
connection  with  the  same  subject.1  Ciesielski 
had  shown,  in  his  study  of  geotropic  move- 
ments, that  roots  extending  horizontally  with 
their  tips  cut  off  did  not  grow  downward.  "  He 
further  states  that,  if  the  tips  are  cut  off  after 
the  roots  have  been  left  extended  horizontally 
for  some  little  time,  but  before  they  have  begun 
to  bend  downwards,  they  may  be  placed  in  any 
position  and  yet  will  bend  as  if  still  acted  on 
by  geotropism ;  and  this  shows  that  some  influ- 
ence had  been  already  transmitted  to  the  bend- 
ing part  from  the  tip  before  it  was  amputated." 
Sachs  repeated  these  experiments,  but  denied 
the  conclusions,  because  in  his  experiments 
the  roots  became  distorted  in  all  directions. 
The  Darwins  undertook  to  learn  the  cause  of 
the  contradiction  in  the  results.  After  describ- 

1  Power  of  Movement  in  Plants,  p.  523. 


68  THE  METHOD    OF  DARWIN. 

ing  unsuccessful  efforts  based  on  reasoning, 
they  go  on  to  say,  "  We  next  thought  that,  if 
care  were  nat  taken  in  cutting  off  the  tips 
transversely,  one  side  of  the  stump  might  be 
irritated  more  than  the  other,  either  at  first,  or 
subsequently  during  the  regeneration  of  the 
tip,  and  that  this  might  cause  the  radicle  to 
bend  to  one  side."  They  amputated  some 
radicles  obliquely  and  some  transversely,  and 
allowed  them  to  grow  perpendicularly.  There 
was  little  or  no  distortion  at  first;  but  after 
two  or  three  days,  when  the  new  tips  began  to 
form,  the  distortion  of  the  obliquely  amputated 
radicles  became  very  conspicuous.  The  new 
tip  was  probably  formed  obliquely,  causing  the 
bending.  Sachs  probably  "  unintentionally  am- 
putated the  radicles  not  strictly  transversely  "  ; 
and  by  not  attending  to  this  apparently  insig- 
nificant condition  he  produced  confusion  and 
failed  to  make  a  discovery. 

This  case  is  interesting  not  only  because  it 
illustrates  the  difficulties  that  are  met  by  the 
individual  investigator,  but  because  it  is  a  typ- 
ical example  of  a  very  large  proportion  of  con- 
tradictions in  results  with  which  the  literature 
of  science  is  burdened.  The  contradiction  in 
the  results  obtained  by  the  two  men  was  due, 
not  to  errors  of  observation,  but  to  neglect  of 


EX  HA  US  TIVENESS.  69 

the  various  conditions  under  which  the  experi- 
ments were  made.  As  I  have  elsewhere  shown 
for  another  more  involved  case,  the  disputant 
observers  were  both  right.1  It  was  lack  of 
exhaustion  of  the  logical  conditions  of  the 
problem  that  led  to  the  contradiction.  Total 
exclusion  of  error  requires  that  every  move- 
ment of  the  experimenter  be  fraught  with  in- 
tention. It  is  fairly  safe  to  assume  that,  if  two 
observers  are  competent  and  upright,  their  con- 
tradictory results,  no  matter  on  what  subject, 
will  prove  essential  to  the  final  solution  of  the 
problem. 

The  publication  of  the  "  Power  of  Movement 
in  Plants "  was  followed  by  several  years  of 
active  investigation  on  and  discussion  of  the 
"Darwinian  curvature"  of  radicles.  It  has 
been  shown  that  radicles,  instead  of  being 
deflected  by  tin-foil  on  soft  sand,  will  pene- 
trate mercury  and  pierce  tin-foil  even  when 
they  strike  it  at  a  quite  high  angle.  In  expla- 
nation of  the  initial  error  of  the  Darwins  it  has 
been  suggested  that  the  radicles  upon  which 
they  experimented  were  wilted.  It  has  been 
further  shown  that  in  the  experiments  in  which 
they  attached  small  hard  objects  to  the  tips  of 
the  radicles  to  induce  them  to  curve,  the  curva- 

1  Popular  Science  Monthly,  January,  1894,  pp.  373~376« 


7O  THE  METHOD   OF  DARWIN. 

ture  was  not  due  to  sensitiveness,  but  to  the 
action  of  the  shellac  by  means  of  which  the 
objects  had  been  attached.  Microscopic  exam- 
ination of  radicles  to  which  the  materials  used 
for  attachment  had  been  applied  showed  that 
the  cells  were  affected.  In  short,  the  curvature 
of  radicles  ascribed  to  sensitiveness  to  touch 
has  been  shown  to  be  due  to  pathological  con- 
ditions brought  into  existence  by  the  experi- 
ments themselves.  The  reasoning  was  correct 
enough,  but  the  premises  were  false.  Initial 
errors  led  to  a  false  conclusion,  but  the  experi- 
ments were  all  valuable  as  starting  points  for 
more  searching  investigations.  The  hypothesis 
of  sensitiveness  has  been  proved  by  Wiesner  1 
and  others  to  be  untenable,  but  much  more  is 
known  of  the  Darwinian  curvature  now  than 
when  the  Darwins  published  their  conclusions. 
All  Darwin's  works  on  plants  furnish  ex- 
amples of  the  practically  complete  develop- 
ment of  the  conditions  of  the  problem.  In 
the  opening  of  the  chapter  on  "  Illegitimate 
Offspring  of  Heterostyled  Plants,"  he  said,  "I 
give  the  results  of  my  experiments  in  detail, 
partly  because  the  observations  are  extremely 

1  Wiesner,  J.,  Untersuchungen  liber  die  Wachsthumsbewe- 
gungen  der  Wurzeln  (Darwinische  und  Geotropische  Wurzel- 
Kriimmung).  Sitz.  Akad.  Wien,  V.  89,  I.  pp.  223-302. 


EXHA  USTIVENESS.  7 1 

troublesome  and  will  not  probably  soon  be 
repeated, —  thus  I  was  compelled  to  count  under 
the  microscope  above  twenty  thousand  seeds  of 
Lythrum  salicaria.  "*  The  whole  of  his  work 
on  the  same  subject  is  on  the  same  scale,  vast 
numbers  of  observations  being  condensed  into 
each  of  numerous  tables. 

It  is  elsewhere  described  how  he  had  fore- 
seen the  importance  of  making  comparative 
observations  on  the  effects  of  cross-  and  self- 
fertilization  in  plants;  and  how  he  had  been 
deterred  by  a  bad  analogy,  and  had  finally  had 
the  subject  thrust  upon  him  while  making 
experiments  on  Linaria  vulgaris  and  the  carna- 
tion with  another  end  in  view.2  There  are 
extant  in  all  biological  literature  few  equally 
fine  examples  of  the  clear  comprehension  of 
the  conditions  of  a  problem  and  of  untiring 
attention  to  them.3  He  began  by  making  a 
preliminary  experiment  with  two  plants,  and, 
finding  that,  as  in  his  previous  accidental 
observations,  the  cross-fertilized  seedling  was 
in  every  respect  superior  to  the  self-fertilized, 
he  proceeded  to  experiment  on  a  very  large 
scale. 

1  Different  Forms  of  Flowers  on  Plants  of  the  same  Species, 
p.  189. 

2  Effects  of  Cross-  and  Self-Fertilization,  p.  8. 
8  Ibid.,  pp.  10-27. 


72  THE  METHOD   OF  DARWIN. 

He  covered  the  plants  with  nets,  and  cross- 
and  self-fertilized  them  artificially  without  cas- 
tration, so  as  to  make  the  cases  parallel  in  all 
respects.  The  seeds  were  thoroughly  ripened; 
the  cross-  and  self-fertilized  seeds  were  chosen 
in  pairs  that  had  germinated  simultaneously, 
and  were  planted  on  opposite  sides  of  the  same 
pot.  When  one  of  a  pair  t  became  sickly  or 
injured,  both  were  thrown  away.  All  the 
seeds  which  remained  after  a  number  of  pairs 
were  thus  planted  were  sown  and  left  to  grow 
up  crowded  on  opposite  sides  of  the  same  pot. 
The  soil  was  carefully  made  uniform ;  the 
plants  on  the  two  sides  of  a  pot  were  always 
watered  at  the  same  time  and  as  equally  as 
possible.  The  plants  on  the  two  sides  of  a  pot 
were  separated  by  a  partition,  but  the  pot  was 
turned  so  that  the  two  sides  would  be  equally 
lighted.  In  this  way  the  cross-  and  self-fer- 
tilized seedlings  that  competed  with  each  other 
were  subjected  to  as  nearly  similar  conditions 
as  human  ingenuity  could  produce.  Different 
sets  of  competing  plants  were  subjected  to 
different  conditions,  to  determine  whether  the 
inequality  between  the  cross-  and  self-fertilized 
seedlings  would  show  itself  only  under  favor- 
able, or  unfavorable,  or  under  all  circumstances. 

In   making   the  comparisons  the  eye  alone 


EX  HA  US  TIVENESS.  7  3 

was  never  trusted.  Many  plants  were  measured 
while  young,  when  nearly  full  grown,  and  when 
matured.  Equal  numbers  of  the  two  kinds 
were  cut  down  and  weighed.  Records  were 
kept  of  the  rate  of  germination,  of  the  periods 
of  flowering,  and  of  productiveness  both  as  to 
the  number  of  capsules  produced  and  as  to  the 
average  number  of  contained  seeds.  Finally, 
the  tables  of  measurements  were  submitted  to 
Francis  Galton  in  order  to  insure  against  error 
and  to  have  them  examined  by  the  best  statis- 
tical methods.  Darwin  intended  at  first  to 
raise  only  one  generation  of  crossed  and  self- 
fertilized  seedlings  of  each  kind,  but  in  many 
cases  he  went  as  high  as  ten  generations. 
Plants  of  different  generations  were  exposed  to 
different  conditions  in  successive  years.  He 
started  crossed  seeds  of  Ipomcea  purpurea  (third 
generation)  forty-eight  hours  later  than  the 
self-fertilized;  seeds  were  sown  outdoors  late 
in  the  season,  and  only  one  stick  given  to  each 
set  to  climb  on ;  two  lots  were  sown  in  a  shady 
and  weedy  part  of  the  garden ;  two  other  lots 
were  sown  in  a  bed  of  candytuft;  seeds  from 
the  same  plant  were  sown,  the  crossed  in  one 
corner  and  the  self-fertilized  in  another  corner 
of  a  tub  in  which  a  Brugmannia  had  been  grow- 
ing, and  in  which  the  soil  was  excessively 


74  THE   METHOD   OF  DARWIN. 

poor;  others  were  transferred  from  the  hot- 
house to  the  coldest  part  of  the  greenhouse;  — 
all  to  test  the  relative  vigor  of  the  crossed  and 
self-fertilized  seedlings. 

He  considered  the  possible  sources  of  error, 
and  showed  that  accidental  cross-fertilization 
of  plants  intended  for  self-fertilization,  and 
accidental  self-fertilization  of  plants  intended 
for  cross-fertilization,  would  diminish  rather 
than  exaggerate  his  results.  It  having  been 
said  that  an  excess  of  pollen  was  injurious,  two 
sets  of  sixty-four  each  of  Ipomcea  purpurea 
were  tested  to  find  whether  the  quantity  of 
pollen  applied  to  the  stigma  made  any  differ- 
ence, and  the  statement  was  proved  untrue. 
He  published  the  details  of  his  experiments, 
because  they  extended  over  eleven  years  and 
"are  not  likely  soon  to  be  repeated."  When 
Darwin  said  in  his  conclusion  that  cross-fertil- 
ized plants  are  superior  to  self-fertilized  plants 
and  have  a  permanent  advantage  over  them  in 
the  struggle  for  existence,  and  that  nature 
abhors  perpetual  self-fertilization,  there  was 
no  man  to  gainsay  it;  his  ingenuity  had  spent 
itself  in  exhausting  the  conditions  of  the  prob- 
lem. He  could  have  reached  the  same  conclu- 
sion from  his  two  accidental  observations  and 
his  first  direct  experiment  on  the  two  plants. 


EX  HA  USTIVENESS.  7  5 

In  this  way  he  could  have  secured  for  himself 
the  priceless  gem  of  "priority  of  discovery  " 
without  the  tedious  years  of  work;  he  could 
then  have  produced  what  so  many  scientists  in 
prominent  positions  produce  on  subjects  fit  to 
occupy  one  mind  for  years,  —  a  few  pages  of 
general  discussion  and  desultory  reference  to 
scattered  and  long-known  facts. 

The  characteristic  of  exhaustiveness  and  its 
consequences  is  well  illustrated  in  his  "detail 
work  "  in  the  Monograph  of  Cirripedia.  By  the 
examination  of  an  enormous  number  of  speci- 
mens he  showed  how  very  variable  are  the  spe- 
cies of  the  genus  Balanus,  and  how,  through 
imperfect  examinations  and  want  of  caution, 
so  many  nominal  species  of  fossil  Balani  have 
been  described.  Discussing  dubious  species, 
he  said,  "  Bronn  does  not  seem  to  have  been 
aware  of  the  absolute  necessity  of  giving 
minute  details  in  his  descriptions  of  fossil 
cirripeds."1  Probably  in  no  department  of 
the  biological  sciences  has  there  been  more 
superficial  and  worthless  work  done  than  in 
the  description  of  species.  This  is  doubtless 
due  to  the  fact  that  a  spurious  fame  could  be 
acquired  by  the  connection  between  the  author's 
name  and  that  of  the  species  he  described. 

1  Monograph  of  the  Cirripedia,  Vol.  II.  pp.  173,  184. 


76  THE   METHOD   OF  DARWIN. 

Even  good  naturalists  have  frequently  regarded 
a  short  description  sufficient  for  ordinary  pur- 
poses of  identification  as  even  preferable  to  a 
minute  enumeration  of  details.  Darwin  not 
only  gave  an  example  of  the  permanent  worth 
of  the  latter  method  of  specific  description  in 
his  "Monograph  of  the  Cirripedia,"  but  his 
book  on  the  "  Origin  of  Species  "  is  one  of  the 
finest  examples  extant  of  the  fact  that  a  short 
statement  of  any  subject  to  be  valuable  and 
forcible  must  be  an  abridgment  of  and  be 
based  upon  a  vast  mass  of  details.  In  a  corre- 
spondence with  Hugh  Strickland  he  expresses 
himself  almost  savagely  in  condemnation  of 
the  wretchedly  poor  work  of  species  describers; 
he  had  unusually  good  reason  to  feel  aggrieved 
because  the  nature  of  his  work  compelled  him 
to  use  so  much  of  the  work  of  others.  But  he 
did  far  less  for  the  improvement  of  specific 
description  by  personal  example  in  the  Mon- 
ograph of  Cirripedia,  and  by  personal  condem- 
nation of  the  poor  work  of  others,  than  he  did 
indirectly  by  his  general  theories  of  descent 
and  natural  selection.  The  doctrine  that  species 
had  their  origin  in  varieties  and  in  individual 
variations  has  changed  the  purpose  of  specific 
description.  Identification  and  classification 
have  been  made  processes  subsidiary  to  some- 


TIME   GIVEN  TO  INVESTIGATIONS.          // 

thing  higher.  The  establishment  of  a  new 
standard  of  value  for  specific  work  has  not  only 
directed  this  work  into  new  channels  and  the 
careful  study  of  details,  but  has  made  many  old 
descriptions  valueless. 

Intimately  connected  with  the  thoroughness 
with  which  scientific  work  is  done  is  the  length 
of  time  spent  on  it.  One  of  the  serious  objec- 
tions to  waiting  for  better  facilities,  more  evi- 
dence, etc.,  when  the  question  of  closing  up  an 
investigation  arises,  is  the  probability  of  loss 
of  interest  in  the  subject.  Promptness  in  com- 
pleting any  line  of  work  seems  commendable 
on  account  of  the  economy  of  time,  the  greater 
certainty  of  recording  results  to  date,  and  the 
importance  of  keeping  the  coast  clear  for  new 
work.  But  work  "completed"  in  a  short  time 
suffers  from  incompleteness,  from  whatever 
point  of  view  it  is  regarded.  Nothing  can  be 
so  demonstrative  as  the  relative  permanence  of 
work  that  has  been  done  slowly  and  work  that 
has  been  done  with  promptness  and  apparent 
vigor.  The  latter  almost  invariably  takes  a 
very  subordinate  place  in  the  literature  of  the 
subject  when  once  that  subject  is  competely 
worked  out. 

In  these  days  of  competition,  when  every 
field  of  biology  is  ferreted  for  new  subjects  of 


78  THE  METHOD   OF  DARWIN. 

investigation,  and  others  are  likely  to  secure 
priority  of  publication,  there  is  every  tempta- 
tion to  publish  prematurely.  Priority  having 
been  gained  by  a  so  called  preliminary  notice, 
a  large  proportion  of  the  subjects  are  dropped 
by  the  original  investigators,  and  let  alone  by 
others  because  they  are  "old."  Time  makes 
investigation  easier.  Where  speed  is  felt  to  be 
necessary,  a  vast  outlay  of  energy  is  frequently 
required  to  discover  what  with  more  time  would 
almost  come  of  itself.  With  the  attention 
steadily  fixed,  time  brings  to  bear  multitudes 
of  facts  that  would  otherwise  be  lost.  Gaps  in 
the  evidence,  if  filled  at  all,  are  too  often  filled 
with  "necessary  inferences"  instead  of  facts. 

There  is  perhaps  no  better  case  on  record  to 
illustrate  the  effect  of  time  on  the  develop- 
ment of  theory  than  the  "  Origin  of  Species/' 
Darwin  had  already  long  reflected  on  the  sub- 
ject when  he  opened  his  first  note-book  for 
facts  in  1837;  and  for  more  than  twenty  years 
thereafter  he  labored  in  analyzing  and  inter- 
preting the  facts  of  nature  by  the  help  of  his 
theory  in  order  to  test  the  latter  in  all  its  rela- 
tions. By  carrying  on  simultaneously  several 
investigations  bearing  on  the  general  subject, 
he  could  let  each  of  his  studies  drag  through 
many  years,  and  yet  was  able  to  accomplish 


TIME  GIVEN  TO  INVESTIGATIONS.        8 1 

much.  Some  of  the  most  important  explana- 
tions under  his  theories  did  not  occur  to  him 
until  years  after  he  had  begun  their  study.  It 
will  be  pointed  out  later  that  after  he  once  got 
possession  of  a  working  hypothesis  his  work 
was  largely  deductive;  and  it  will  be  shown 
how  extremely  difficult  it  is  to  work  out  all  the 
important  consequences  of  such  a  hypothesis 
even  in  many  years.  After  it  is  once  done  the 
task  seems  so  easy  that  the  wonder  is  that  it 
was  not  done  sooner.  But  the  contemplation 
of  the  development  of  a  great  theory  soon  re- 
veals the  enormous  difficulties  in  the  way  of 
one  or  many  who  seek  to  work  out  its  conse- 
quences. Darwin  did  in  each  of  his  investiga- 
tions what  is  usually  done  for  a  subject  by  a 
number  of  successive  workers ;  each  makes  an 
important  contribution  to  the  subject,  removes 
a  serious  objection  to  a  theory,  explains  a  sec- 
tion of  the  evidence,  points  out  an  important 
consequence,  or  modifies  the  statement  of  it 
to  bring  it  more  clearly  in  harmony  with  the 
greater  knowledge  on  the  subject.  By  succes- 
sive approximations  many  men,  working  toward 
the  same  end,  originate,  build  up,  and  improve 
a  theory  until  it  takes  its  place  among  perma- 
nently established  truths.  As  far  as  it  was 
possible  for  one  man  to  do  so  Darwin  did  all 


-g  THE  METHOD   OF  DARWIN, 

this  for  his  subjects  before  he  gave  them  to  the 
world.  His  work  on  the  "  Expression  of  the 
Emotions"  began  in  1838  and  closed  in  1872; 
"Insectivorous  Plants,"  1860-1876;  "Vegetable 
Mould  and  Earthworms,"  1837-1881.  One  of 
the  most  notable  legacies  that  he  left  to  the 
ambitious  student  is  his  example  of  great  energy 
and  great  patience,  his  incarnation  of  the  truth 
that  time,  as  well  as  reason,  is  the  handmaid  of 
science. 

Coupled  with  the  habit  of  treating  exhaust- 
ively the  subjects  with  which  he  dealt,  and 
the  willingness  to  bide  his  time  for  publication 
until  his  views  had  reached  their  full  maturity, 
was  his  extreme  conscientiousness  in  giving 
full  force  to  the  objections  against  his.  general 
results.  The  habit  of  pursuing  all  the  facts  to 
their  meaning  made  it  possible  for  him  to  say 
that  he  had  been  able  to  consider  in  advance 
all  the  objections  that  were  afterwards  made 
to  any  of  his  views.  The  summing  up  of  his 
experiments  on  Adonis  czstivalis,  in  the  "  Effects 
of  Cross-  and  Self-Fertilization,"  is  an  example 
of  the  great  length  to  which  he  went  in  record- 
ing facts,  especially  if  they  were  in  any  way 
opposed  to  his  conclusions.  He  said,  "The 
results  of  my  experiments  on  this  plant  are 
hardly  worth  giving,  as  I  remarked  in  my  notes 


TREATMENT  OF  OBJECTIONS.  8 1 

made  at  the  time,  *  seedlings  from  some  un- 
known cause,  all  miserably  unhealthy, '  nor  did 
they  ever  become  healthy;  yet  I  feel  bound  to 
give  the  present  case,  as  it  is  opposed  to  the 
general  results  at  which  I  have  arrived."1 

He  did  not  hesitate  to  diminish  the  positive 
results  of  his  experiments  or  the  effect  of  his 
views  by  incorporating  all  exceptions,  unless 
they  were  clearly  due  to  some  known  extra- 
neous cause.  But  his  inability  to  leave  any 
thing  unexplained  was  so  great  that  he  rarely 
left  exceptional  facts  without  at  least  sugges- 
tions of  possible  explanations.  He  was  ex- 
tremely ingenious  in  guessing  explanations  for 
facts  that  could  not  be  brought  under  the  same 
general  explanation  as  the  other  facts  of  their 
class.  A  good  instance  of  this  art  of  wriggling 
is  his  attempt  to  explain  the  sloping  terraces 
of  Coquimbo.2 

1  Effects  of  Gross-  and  Self-Fertilization,  p.  128. 

2  Geological  Observations,  etc.,  pp.  256-258. 


V. 

NEGATIVE  EVIDENCE. 

IT  would  naturally  be  expected,  from  Dar- 
win's clear  notions  of  evidence  in  gen- 
eral, and  the  necessity  that  he  was  under  all 
his  life  of  handling  vast  bodies  of  complicated 
evidence,  that  his  work  would  furnish  examples 
for  the  treatment  of  negative  evidence.  His 
estimate  of  its  value  is  well  shown  by  his  treat- 
ment of  the  question  whether  Primula  veris, 
P.  vttlgaris,  and  P.  elatior  are  different  forms 
of  the  same  species.1  After  discussing  the 
evidence  in  favor  of  this  view  he  said,  "  Nega- 
tive evidence  is  of  little  value;  but  the  follow- 
ing facts  may  be  worth  giving."  Then  follows 
the  recital  of  his  efforts  to  determine  whether 
the  cowslip  varies  enough  to  justify  the  belief. 
He  transplanted  cowslips  from  the  fields  into  a 
shrubbery,  and  then  into  highly-manured  land; 
the  next  year  they  were  protected  from  insects, 
artificially  fertilized,  and  seed  grown,  which 

1  Different    Forms  of    Flowers   on   Plants    of   the    same 
Species,  p.  62. 


NEGATIVE  EVIDENCE.  83 

was  sown  in  a  hot-bed.  The  young  plants 
were  set  out,  some  in  very  rich  soil,  some  in 
stiff,  poor  clay,  some  in  old  peat,  and  others  in 
pots  in  the  greenhouse,  —  seven  hundred  and 
sixty-five  in  all.  Though  they  and  their  parents 
were  subjected  to  all  this  diversity  of  treat- 
ment, "not  one  of  them  presented  the  least 
variation  except  in  size."  Negative  evidence 
is  indeed  of  little  value,  unless  it  can  be  shown 
that  it  covers  the  whole  ground.  In  order  to 
transform  these  experiments  into  proof  it  would 
be  necessary  to  show  that,  if  the  three  forms 
belong  to  one  species,  the  cowslip  should  have 
varied  under  the  conditions  to  which  it  was 
subjected.  It  is  far  more  difficult  to  disprove 
a  proposition  by  negative  evidence  than  by 
proving  the  truth  of  its  contradictory.  Darwin 
accordingly  demonstrated  what  several  other 
botanists  had  surmised:  that  the  oxlip  is  a 
hybrid  between  the  cowslip  and  the  primrose. 

His  efforts  to  determine  whether  Orchis  morio 
secretes  nectar  also  furnish  a  good  illustration 
of  his  treatment  of  negative  evidence.1  A 
nectary  implies  nectar,  but  Sprengel  had  thor- 
oughly searched  many  flowers  of  O.  maculata 
and  morio,  and  could  not  find  a  drop.  Of 
his  own  efforts  in  this  direction,  Darwin  said, 

1  Fertilization  of  Orchids,  pp.  36-41. 


84  THE  METHOD   OF  DARWIN. 

"  I  have  looked  to  all  our  common  British 
species  and  could  find  no  trace  of  nectar;  I 
examined,  for  instance,  eleven  flowers  of  O. 
maculata,  taken  from  different  plants  growing 
in  different  districts,  and  taken  from  the  most 
favorable  position  on  the  spike,  and  could  not 
find  under  the  microscope  the  smallest  bead  of 
nectar."  Sprengel  believed  that  these  plants 
exhibit  an  organized  system  of  deception,  "for 
he  well  knew  that  the  visits  of  insects  were  in- 
dispensable for  their  fertilization  "  ;  but  Darwin 
could  not  believe  in  so  gigantic  an  imposture. 
"Notwithstanding  these  several  facts,"  he  went 
on,  "  I  still  suspected  that  nectar  must  be 
secreted  by  our  own  orchids,  and  I  determined 
to  examine  O.  morio  rigorously.  As  soon  as 
the  flowers  were  open  I  began  to  examine  them 
for  twenty-three  consecutive  days;  I  looked  at 
them  after  hot  sunshine,  after  rain,  and  at  all 
hours;  I  kept  the  spikes  in  water  and  examined 
them  at  midnight  and  early  the  next  morning." 
He  irritated  the  nectaries  with  bristles  and  ex- 
posed them  to  irritating*vapors.  He  examined 
flowers  whose  pollinia  had  been  removed,  and 
others  which  would  probably  have  them  soon 
removed.  But  the  nectary  was  invariably  dry. 
Only  after  he  had  made  the  negative  evidence 
as  complete  as  it  could  be  made,  by  examining 


NEGATIVE  EVIDENCE.  85 

the  nectaries  of  very  many  flowers  from  differ- 
ent places  under  all  the  possible  circumstances 
in  which  nectar  might  be  secreted,  did  he  feel 
justified  in  saying,  "We  may  therefore  safely 
conclude  that  the  nectaries  of  the  above  named 
orchids  neither  in  this  country  nor  in  Germany 
ever  contain  nectar."  Even  then  he  restricted 
the  negative  conclusion  to  the  two  countries 
in  which  the  exhaustive  examinations  had  been 
made. 

But  he  did  not  rest  with  this  negative  evi- 
dence. It  was  strong  enough  to  convince  him 
that  there  was  no  ordinary  nectar,  but  the 
further  evidence  that  he  presents  shows  how 
quickly  negative  evidence  falls  into  the  back- 
ground in  the  presence  of  even  the  most  indirect 
positive  evidence.  He  was  thoroughly  con- 
vinced that  these  orchids  require  the  visits  of 
insects  for  fertilization,  that  insects  visit  flowers 
for  the  attractions  offered  in  the  way  of  nec- 
tar, pollen,  etc.  ;  that  nature  could  not  deceive 
insects  by  a  permanent  imposture,  and  yet  that 
in  these  orchids  the  ordinary  attraction  was 
absent.  It  was  as  if  a  crime  had  been  com- 
mitted, and  he  were  asked  to  believe  there  was 
no  criminal. 

In  examining  the  nectaries  of  the  several 
species  of  orchids  he  was  "surprised  at  the 


86  THE  METHOD  'OF  DARWIN. 

degree  to  which  the  inner  and  outer  membranes 
forming  the  tube  or  spur  were  separated  from 
each  other,  also  at  the  delicate  nature  of  the 
inner  membrane,  .  .  .  and,  lastly,  at  the  quan- 
tity of  fluid  contained  between  the  two  mem- 
branes." He  found  the  space  between  the 
membranes  of  other  nectaries  quite  dry.  He 
then  examined  other  forms  that  do  secrete  nec- 
tar in  the  ordinary  way,  and  found  the  mem- 
branes closely  united,  instead  of  separated  by 
a  space.  "I  was  therefore  led  to  conclude,"  he 
said,  "  that  insects  penetrate  the  lax  membrane 
of  the  nectaries  of  the  above  named  orchids  and 
suck  the  copious  fluid  between  the  two  mem- 
branes. This  was  a  bold  hypothesis,  for  at  the 
time  no  case  was  known  of  insects  penetrating 
with  their  proboscides  even  the  laxest  mem- 
brane." He  afterwards  learned  that  at  the 
Cape  of  Good  Hope  moths  and  butterflies  pene- 
trate peaches  and  plums,  and  in  Queensland, 
Australia,  a  moth  penetrates  the  rind  of  the 
orange.  These  facts  merely  proved  his  antici- 
pation less  anomalous  than  it  had  seemed. 
The  bees  which  he  saw  visiting  Orchis  morio 
kept  their  proboscides  inserted  in  the  nectaries 
for  some  time.  He  opened  several  nectaries, 
and  found  brown  specks,  due,  as  he  believed, 
to  punctures  made  some  time  before.  Herman 


NEGATIVE  EVIDENCE.  87 

Miiller  has  since  corroborated  Darwin's  inter- 
pretation, saying,  "  My  own  observations  have 
confirmed  this  view,  as  well  as  every  detail  of 
the  rest  of  Darwin's  account."1  The  nega- 
tive evidence  was,  by  its  very  completeness,  a 
stumbling-block  to  Darwin's  beliefs.  As  must 
sooner  or  later  be  done  with  all  instances  of 
negative  evidence,  he  again  set  about  replacing 
it  by  positive  evidence,  which  removed  the 
necessity  of  the  belief  which  the  negative  evi- 
dence destroyed.  Until  that  was  done  the  neg- 
ative evidence  increased  rather  than  diminished 
the  mystery  that  needed  solution. 

1  H.  Miiller,  Fertilization  of  Flowers,  pp.  535-538. 


VI. 

CLASSIFICATION. 

T)ERFORMED  consciously  or  unconsciously, 
•*-  the  act  of  classification  is  indispensable 
to  and  accompanies  every  scientific  inference. 
A  mind  is  orderly  or  slovenly,  according  as  it 
does  or  does  not  habitually  and  accurately 
classify  the  facts  with  which  it  comes  in  con- 
tact. The  success  of  an  investigation,  the 
worth  of  a  conclusion,  are  in  direct  proportion 
to  the  fidelity  to  this  principle  and  the  exhaust  - 
iveness  with  which  the  process  is  carried  out. 

In  nature,  constant  forces  at  work  upon  vary- 
ing materials  necessarily  produce  segregation; 
the  like  are  brought  together,  and  the  unlike 
separated.  The  result  is  a  literally  "natural 
classification."  This  simple  result  is  rarely 
realized.  The  forces  at  work  are  so  numerous, 
and  have  acted  so  long,  that,  especially  with 
reference  to  living  things,  Nature's  serial  clas- 
sifications in  space  and  time  have  been  broken 
up  and  thrown  into  confusion  to  such  an  extent 
that  they  are  seldom  recognizable.  The  result 


CLASSIFICATION.  89 

is  a  superficial  chaos  of  phenomena.  The 
recognition  of  natural  classifications  was  an 
excessively  slow  growth;  they  were  finally 
worked  out  by  the  slow  collection  of  material 
and  successive  attempts  at  a  natural  arrange- 
ment. In  nature's  arrangement  of  living  things 
over  the  earth  it  has  been  very  difficult  to  recog- 
nize law,  and  at  first  it  was  possible  only  where 
isolation  has  been  long  continued  and  the  forces 
at  work  upon  living  things  have  been  few  and 
steady  in  their  action.  Even  then  the  recog- 
nition has  required  extensive  travel  and  a  power- 
ful inclination  to  classify  and  recognize  the 
relations  of  distant  facts  to  each  other. 

It  was  the  recognition  of  several  such  arrange- 
rrients  or  classifications  in  Nature  that  first  led 
Darwin  to  reflect  on  the  Origin  of  Species. 
What  immortalized  his  observations  is  not  the 
simple  fact  that  they  were  made,  but  that  by 
their  cumulative  presentation  they  led  Darwin 
to  seek  an  adequate  cause  for  these  natural 
arrangements. 

After  pointing  out,  in  the  narrative  of  his 
voyage,  the  striking  relation  between  the  fossil 
and  the  living  animals  of  South  America,  he 
said  :  "  This  wonderful  relationship  in  the  same 
continent  between  the  dead  and  the  living  will, 
I  do  not  doubt,  hereafter  throw  more  light  on 


go  THE  METHOD   OF  DARWIN. 

the  appearance  of  organic  beings  on  our  earth, 
and  their  disappearance  from  it,  than  any  other 
class  of  facts."1  In  passing  southward  over 
the  continent  of  South  America,  he  recognized 
another  of  Nature's  classifications:  he  noticed 
the  frequent  recurrence  of  the  fact  that  a  species 
occupying  a  given  region  was  replaced  to  the 
southward  by  a  closely  related  species,  and  this 
serial  arrangement  impressed  itself  strongly 
upon  him.  His  visit  to  the  Galapagos  Islands 
gave  him  an  almost  perfect  example  of  sim- 
plicity in  the  working  of  Nature's  forces.  The 
conditions  for  one  of  Nature's  classifications 
were  perfect.  When  Lawson,  the  Vice  Gover- 
nor, had  declared  to  him  that  the  tortoises  from 
the  different  islands  differed  from  one  another, 
Darwin  did  not  see  the  significance  of  the  fact. 
He  mixed  up  his  collections  from  the  various 
islands,  and  did  not  dream  that  there  lay  before 
him  one  of  the  most  remarkable  facts  that 
Nature  ever  revealed  to  a  naturalist.  By  some 
happy  accident  he  compared  the  many  speci- 
mens of  mocking  thrushes  shot  on  Charles 
Island  with  those  from  Albemarle  Island,  and 
was  astonished  to  find  that  they  belonged  to 
different  species.  It  was  not  the  fact  that 
there  were  two  species  of  mocking  thrush  liv- 

1  Naturalist's  Voyage  around  the  World,  p.  173. 


CLASSIFICA  TION.  9 1 

ing  together  on  the  two  islands,  but  that  they 
lived  apart,  each  on  its  own  island,  and  that 
they  were  closely  instead  of  distantly  related, 
—  that  several  islands  were  stocked,  each  with 
its  own  species,  or  perhaps  variety,  of  the  same 
kind  of  animal,  — that  struck  him  with  wonder. 
These  facts  haunted  him,  and  drove  him  to  look 
for  others  like  them.  Upon  his  arrival  at  home 
his  insect  collection  proved  the  same  law.  He 
had  fortunately  kept  the  plants  of  the  different 
islands  separate,  and  Hooker,  at  Darwin's  re- 
quest to  see  whether  the  law  held  good  for  them, 
found  it  to  be  so.1  Darwin  had  unwittingly 
carried  one  of  Nature's  beautiful  classifications 
home  to  England  with  him.  Keen  insight  into 
the  relation  of  facts  to  one  another  had  enabled 
him  to  recognize  three  striking  examples  of 
Nature's  arrangements. 

The  habit  of  grouping  facts  to  extract  the 
truth  from  them  was  indispensable  to  Darwin's 
work,  for  he  constantly  dealt  with  large  bodies 
of  facts  that  were  manageable  in  no  other  way. 
It  will  be  seen  how  difficult  it  is  even  for  a 
powerful  observer  to  see  facts  for  which  he  is 
not  looking,  even  though  they  lie  under  his 
feet.  An  act  of  classification,  to  be  worth 
much,  must  usually  be  an  effort  to  answer 

1  Naturalist's  Voyage  around  the  World,  pp.  393-398. 


92  THE  METHOD   OF  DARWIN. 

a  direct  question.  Science  has  derived  very 
little  or  no  benefit  from  the  miscellaneous  col- 
lecting and  grouping  of  facts  without  any  pre- 
vious notion  of  what  they  are  likely  to  reveal. 
An  investigation  is  usually  made  for  the  pur- 
pose of  answering  a  definite  question,  or  of 
verifying  an  anticipation.  With  some  such 
end  in  view,  with  some  principle  by  which  the 
classification  is  guided,  the  result  usually  re- 
veals not  only  what  was  looked  for,  but  fre- 
quently still  more  fundamental  characteristics ; 
for  it  is  impossible  to  throw  facts  into  any 
order  which  reveals  one  truth  without  dragging 
others  into  the  light  with  it.  The  character  of 
Darwin's  work  required  constant  recourse  to 
lists  and  tables;  he  appreciated  fully  both  their 
value  and  their  treachery,  and  his  great  ability 
to  recognize  all  the  points  brought  out,  no  mat- 
ter whether  he  was  looking  for  them  or  whether 
they  bore  directly  on  the  subject  which  he  hap- 
pened to  be  investigating  or  not,  made  them 
enormously  useful  to  him. 

Darwin's  original  purpose  in  measuring  the 
heights  of  the  gravel-capped  plains  of  Patagonia 
was  to  ascertain  the  heights  at  which  recent 
fossil  shells  occurred.  These  measurements 
gave  him  all  he  sought,  —  a  notion  of  the  amount 
of  elevation  in  the  recent  period.  On  compar- 


CLASS 'IFICA  TTON.  93 

ing  his  measurements  with  those  of  the  Beagle 
survey,  he  was  struck  with  their  uniformity. 
He  tabulated  all  the  measurements  represent- 
ing the  summit  edges  of  the  plains;  and  the 
tabulation  proved  to  him  that  the  elevation  of 
the  land  had  gone  on  at  a  remarkably  equable 
rate  over  a  north  and  south  distance  of  at  least 
five  hundred  miles.1  There  is  comparatively 
little  danger  of  throwing  away  effort  in  a  well 
directed  classification.  The  danger  lies  in  not 
comprehending  the  vast  significance  of  the 
process  both  in  actual  investigation  and  in  the 
presentation  of  results,  and  in  the  lack  of  per- 
sistent determination  to  exhaust  its  resources. 
Darwin  himself  sometimes  owed  it  to  happy 
accident  that  he  did  not  overlook  this  powerful 
instrument. 

Guided  by  deduction  to  the  probable  relation 
of  the  distribution  of  volcanoes  to  that  of  coral 
islands,  and  to  the  distribution  of  fringing  and 
barrier  reefs  and  atolls  in  relation  to  each  other, 
he  spent  months  in  mapping  them  from  the 
descriptions  of  voyagers,  surveying  vessels,  etc. 
From  the  classification  of  a  vast  chaotic  mass 
of  facts  scattered  throughout  geographic  and 
geological  literature,  he  extracted  some  of  the 
most  important  conclusions  of  his  whole  work 

1  Geological  Observations,  etc.,  p.  211. 


94  THE  METHOD   OF  DARWIN. 

on  coral  islands.1  Time  has  shown  that  the 
conclusions  reached  from  this  mapping  of  facts 
is  too  general.  The  conclusions  that  volcanoes 
are  invariably  absent  from  the  areas  which  have 
recently  subsided  or  are  still  subsiding,  and  are 
commonly  present  in  areas  that  are  rising  or 
have  recently  risen,  that  fringing  reefs  lie  in 
the  areas  of  elevation  and  atolls  in  the  areas 
of  subsidence,  may  not  be  accepted  without  im- 
portant reservations.  But  whether  time  shall 
ultimately  substantiate  or  correct  Darwin's  con- 
clusions, or  shall  even  destroy  some  of  them, 
his  classifications  will  always  remain  essential 
to  the  study  of  coral  islands. 

1  Structure  and  Distribution  of  Coral  Islands,  p.  189. 


VII. 

ANALOGY. 

A  NALOGICAL  reasoning  plays  a  very  im- 
<L±  portant  part  in  all  scientific  work;  and 
Darwin  frequently  availed  himself  of  its  help 
in  making  discoveries  and  establishing  conclu- 
sions. He  used  every  logical  device  to  estab- 
lish and  extend  his  theories,  and  there  is  no 
lack  of  material  from  which  to  choose  exam- 
ples. But  analogy,  when  used  on  a  large  scale, 
proves  so  treacherous,  that  it  is  useful  for  the 
most  part  only  in  giving  clues  to  discoveries. 
There  are  but  few  examples  of  analogical  rea- 
soning on  a  large  scale  in  Darwin's  works. 
The  most  important,  perhaps,  is  his  work  on 
Insectivorous  Plants. 

It  has  already  been  told  how,  while  resting 
at  Hartfield  after  years  of  labor  on  the  Origin 
of  Species,  he  was  struck  by  the  number  of 
insects  caught  by  the  leaves  of  the  common 
sundew.  It  soon  became  evident  to  him  that 
"  Droseraw&s  excellently  adapted  for  the  special 
purpose  of  catching  insects,  so  that  the  subject 


96  THE  METHOD   OF  DARWIN. 

seemed  well  worthy  of  investigation."1  As 
soon  as  he  began  to  work  on  Drosera,  and  was 
led  to  believe  that  the  leaves  absorbed  nutri- 
tious matter  from  the  insects,  he  began  to 
reason  by  analogy  from  the  well  understood 
digestive  capacity  of  animals.  One  needs  but 
to  imagine  an  attempt  to  do  the  work  without 
any  knowledge  of  animal  digestion  to  under- 
stand at  once  its  impossibility  under  such  con- 
ditions. By  connecting  his  observations  with 
the  well  known  animal  processes  he  proceeded 
on  a  course  of  rapid  discovery  that  must  other- 
wise have  remained  entirely  closed  to  him.  At 
almost  every  step  he  drew  suggestions  from 
and  checked  his  results  by  reference  to  animal 
digestion.  He  made  preliminary  "crucial" 
experiments  by  immersing  some  leaves  of 
Drosera  in  nitrogenous  and  others  in  non- 
nitrogenous  fluids  of  the  same  density  to  deter- 
mine positively  whether  the  former  affected 
the  leaves  differently  from  the  latter.2  The 
discovery  that  Drosera  detects  "with  almost 
unerring  certainty  the  presence  of  nitrogen " 
in  various  fluids  "led  me  to  inquire,"  he  said, 
''whether  Drosera  possessed  the  power  of  dis- 
solving solid  animal  matter;  the  experiments 
proving  that  the  leaves  are  capable  of  true 

J  Insectivorous  Plants,  p.  2.  2  Ibid.,  p.  76. 


ANALOGY.  97 

digestion,  and  that  the  glands  absorb  the 
digested  matter."1  "These  are,  perhaps,  the 
most  interesting  of  all  my  observations  on 
Drosera,  as  no  such  power  was  before  distinctly 
known  to  exist  in  the  vegetable  kingdom." 
Having  by  analogy  established  the  power  of 
true  digestion  in  plants,  analogy  led  him  to 
seek  in  plants  the  elements  that  do  the  work 
of  digestion  in  animals.  Bringing  together 
what  was  known  of  plants,  he  pointed  out  that 
the  juices  of  many  plants  contain  an  acid,  and 
so  one  element  of  a  digestive  fluid  was  at  hand; 
and  that  all  plants  possess  the  power  of  dis- 
solving albuminous  or  proteid  substances,  pro- 
toplasm, chlorophyll,  etc.,  and  that  "this  must 
be  effected  by  a  solvent,  probably  consisting 
of  a  ferment  together  with  an  acid."2  After 
writing  the  last  quoted  sentence  he  learned 
that  a  ferment  which  converted  albuminous  sub- 
stances into  true  peptones  had  been  extracted 
from  the  seeds  of  the  vetch. 

Sachs  mentioned  the  discovery  of  the  fer- 
ment, recorded  the  fact  that  peptones  had 
themselves  been  actually  found  in  the  seeds  of 
the  lupine,  and  added  "as  we  come  to  know 
the  proteinaceous  reserve  materials  of  plants 
better,  and  if  we  follow  their  behavior  in  the 

1  Insectivorous  Plants,  p.  268.  2  Ibid.,  p.  362. 

7 


98  THE  METHOD   OF  DARWIN. 

animal  body  also,  it  can  scarcely  be  doubtful 
that,  in  spite  of  incomplete  knowledge,  the 
assumption  is  nevertheless  warranted  that  pep- 
tonizing  ferments  are  perhaps  universally  dis- 
tributed in  plants."1  "Attention  was  first 
drawn  to  the  occurrence  of  peptonizing  fer- 
ments in  the  vegetable  kingdom  by  the  re- 
markable phenomena  observed  in  the  so  called 
insectivorous  plants."  By  analogical  reasoning 
a  whole  new  field  of  study  was  opened ;  a  new 
view  of  the  powers  of  plants  was  gained,  and  a 
much  closer  analogy  between  plant  and  animal 
functions  was  established.  But  if  recent  studies 
are  taken  into  account,  the  question  may  be 
raised  whether  this  stupendous  analogical  struc- 
ture has  not  been  undermined.  Tischutkin 
contends  that  the  "digestion"  of  insectivorous 
plants  is  not  accomplished  in  the  same  way  as 
in  animals,  but  is  due  to  bacteria;  that  the 
pepsin  of  the  leaves  is  not  a  secretion  of  the 
plant,  but  a  by-product  of  the  activity  of  the 
bacteria.2  He  proves  that  bacteria  capable  of 
dissolving  egg  albumen  are  always  present  in 
the  secretion  of  the  leaves;  that  they  come 
principally  from  the  air,  that  the  plant  only 
furnishes  a  medium  for  them  to  live  in,  that 

1  Sachs,  Physiology  of  Plants,  p.  344. 

2  Berichte  der  Deutschen  Botanischen  Gesellschaft,  1889. 


ANALOGY.  99 

disintegration  of  albuminous  substances  begins 
only  after  enough  micro-organisms  are  devel- 
oped to  do  the  work,  and  that  the  plant  simply 
assimilates  what  these  lower  organisms  have 
set  free.  The  relation  between  the  insectiv- 
orous plants  and  the  bacteria  is  one  of  genuine 
symbiosis. 

If  the  whole  of  Tischutkin's  contention  is 
true,  the  great  body  of  facts  brought  out  by 
Darwin  must  still  be  placed  to  the  credit  of 
analogical  reasoning.  The  facts  concerning 
plant  and  animal  digestion  would  still  remain 
parallel,  both  in  the  succession  of  the  phenom- 
ena and  in  the  results.  It  would  be  another 
illustration  of  the  vast  importance  of  analogy 
in  scientific  method,  and  of  the  fact  that  every 
analogy,  the  strongest  as  well  as  the  faintest, 
will  sooner  or  later  fail. 

In  another  instance,  analogical  reasoning 
from  animals  to  plants  actually  deterred  him 
from  discovering  the  truth  to  which  other  logi- 
cal processes  might  have  led  him.  He  states 
the  case  so  clearly  himself  that  it  will  almost 
suffice  to  quote  him.1  "The  adaptation  of 
flowers  for  cross-fertilization  is  a  subject 
which  has  interested  me  for  the  last  thirty- 
seven  years.  .  .  .  From  my  own  observations 
1  Effects  of  Cross-  and  Self-Fertilization,  pp.  6-8. 


100  THE  METHOD   OF  DARWIN. 

on  plants,  guided,  to  a  certain  extent,  by  the 
experience  of  breeders  of  animals,  I  became 
convinced  many  years  ago  that  it  is  a  general 
law  of  nature  that  flowers  are  adapted  to  be 
crossed,  at  least  occasionally,  by  pollen  from 
a  distinct  plant."  It  was  a  direct  deduction 
from  his  theory  of  natural  selection  that,  since 
they  are  adapted  for  cross-fertilization,  cross- 
fertilization  must  be  advantageous  to  them. 
Hence  it  was  perfectly  natural  that  he  should 
like  to  verify  it.  "It  often  occurred  to  me," 
he  said,  "that  it  would  be  advisable  to  try 
whether  seedlings  from  cross-fertilized  flowers 
were  in  any  way  superior  to  those  from  self- 
fertilized  flowers.  But  as  no  instance  was 
known  with  animals  of  any  evil  appearing  in 
a  single  generation  from  the  closest  possible 
interbreeding,  that  is,  between  brothers  and 
sisters,  I  thought  that  the  same  rule  would 
hold  good  with  plants,  and  that  it  would  be 
necessary,  at  the  sacrifice  of  too  much  time,  to 
self-fertilize  and  intercross  plants  during  sev- 
eral successive  generations,  in  order  to  arrive 
at  any  results.  I  ought  to  have  reflected  that 
such  elaborate  provisions  favoring  cross-fertili- 
zation as  we  see  in  innumerable  plants  would 
not  have  been  acquired  for  the  sake  of  a  dis- 
tant and  slight  advantage,  or  of  avoiding  a 


ANALOGY.  10 1 

distant  and  slight  evil.  Moreover,  the  fertili- 
zation of  a  flower  by  its  own  pollen  corresponds 
to  a  closer  form  of  interbreeding  than  is  pos- 
sible with  ordinary  bisexual  animals;  so  that 
an  earlier  result  might  have  been  expected." 

He  had  carried  the  deduction  far  enough  to 
warrant  an  effort  to  verify  it,  but  was  deterred 
by  analogical  reasoning  from  pursuing  the 
matter  further.  Had  he  clung  to  his  general 
theory  and  the  special  facts  to  be  explained 
under  it,  he  would,  as  he  said  himself,  have 
reached  an  early  result.  The  analogy,  if  it 
had  served  a  good  purpose,  ought  to  have  led 
him  to  reason  that  since  continuous  interbreed- 
ing is  harmful  among  animals,  although  there 
are  no  special  adaptations  to  prevent  it,  or  to- 
encourage  the  opposite,  then,  surely,  the  harm- 
ful effects  of  close  breeding  and  the  benefits  of 
cross-fertilization  ought  to  be  very  marked  in 
plants,  with  their  striking  adaptation  for  cross- 
fertilization.  The  analogy  had  clearly  led  him 
astray ;  and  he  was  finally  brought  back  to  the 
subject  by  a  different  route. 

"  I  was  at  last  led  to  make  the  experiments 
recorded  in  the  present  volume  from  the  follow- 
ing circumstance.  For  the  sake  of  determin- 
ing certain  points  with  respect  to  inheritance, 
and  without  any  thought  of  the  effects  of  close 


102  THE  METHOD   OF  DARWIN. 

interbreeding,  I  raised,  close  together,  two 
large  beds  of  self -fertilized  and  crossed  seed- 
lings from  the  same  plant  of  Linaria  vulgaris 
(common  Toad  Flax).  To  my  surprise,  the 
crossed  plants,  when  fully  grown,  were  plainly 
taller  and  more  vigorous  than  the  self-fertilized 
ones. 

"Bees  incessantly  visit  the  flowers  of  this 
Linaria,  and  carry  pollen  from  one  to  the 
other;  and  if  the  insects  are  excluded  the 
flowers  produce  extremely  few  seeds,  so  that 
the  wild  plants  from  which  my  seedlings  were 
raised  must  have  been  intercrossed  during  all 
previous  generations.  It  seems  therefore  quite 
incredible  that  the  difference  between  the  two 
beds  of  seedlings  could  have  been  due  to  a 
single  act  of  self-fertilization;  and  I  attributed 
the  result  to  the  self-fertilized  seeds  not  hav- 
ing been  well  ripened,  improbable  as  it  was 
that  all  should  have  been  in  this  state,  or  to 
some  other  accidental  and  inexplicable  cause." 
During  the  next  season  he  raised  two  beds  of 
carnations  (Dianthus  caryophyllus)  in  the  same 
way  and  for  the  same  purpose;  the  preceding 
generations  in  this  case  also  must  have  been 
continuously  cross-fertilized;  again  "the  self- 
fertilized  seedlings  were  plainly  inferior  in 
height  and  vigor  to  the  crossed." 


ANALOGY.  IO3 

"  My  attention, "  he  said,  "  was  now  thoroughly 
aroused,  for  I  could  hardly  doubt  that  the  dif- 
ference between  the  two  beds  was  due  to  the 
one  being  the  offspring  of  crossed  and  the  other 
of  self-fertilized  flowers."  After  the  effects 
of  cross-  and  self-fertilization  had  been  thus 
thrust  upon  him,  he  proceeded  to  make  the 
exhaustive  examination  that  ran  through  many 
years,  and  finally  filled  a  volume.  He  foresaw 
the  meaning  of  the  adaptations  for  cross-fer- 
tilization and  the  character  of  the  results,  but 
was  deterred  by  a  false  analogy  from  making 
the  observations  to  which  a  careful  study  of  the 
facts  by  themselves  would  have  infallibly  led 
him;  and  was  finally  driven  to  the  subject 
again  by  the  empirical  observation  of  the  facts 
that  he  had  anticipated  by  reasoning.  It  may 
seem  strange  that  the  very  consequences  which 
theory  led  him  to  expect  had  to  be  twice  forced 
upon  the  attention  of  one  who  was  so  quick  to 
seize  Nature's  suggestions,  before  he  could  be 
brought  to  investigate  them.  But  the  strange- 
ness of  such  an  intellectual  phenomenon  is  all 
due  to  the  afterthought.  Even  in  the  sciences 
that  are  most  rigidly  deductive  it  is  a  common 
thing  for  the  investigator  to  stumble  indirectly 
upon  results  which  he  might  have  foreseen  or 
often  did  more  or  less  perfectly  foresee. 


104  THE  METHOD   OF  DARWIN. 

In  a  more  complex  case,  analogy  led  to  a  con- 
clusion which,  although  it  could  not  be  verified, 
possesses  great  importance  in  relation  to  one  of 
the  principal  difficulties  in  the  way  of  the  gen- 
eral theory  of  natural  selection.  In  the  course 
of  the  investigation  on  "  Different  Forms  of 
Flowers  on  Plants  of  the  same  Species,"  he 
noticed  the  striking  parallelism  between  the 
phenomena  of  hybridism  and  those  of  the 
heterostyled  plants  which  he  was  studying.1 
When  once  the  parallelism  was  established, 
the  remarkable  and  puzzling  facts  of  hybridism 
doubtless  furnished  a  solid  analogical  basis 
from  which  to  foresee  and  scrutinize  the  results 
of  crossing  the  different  forms  of  heterostyled 
plants. 

Difficulty  in  uniting  two  forms  and  sterility 
of  their  offspring  had  been  almost  universal-ly 
regarded  as  a  test  of  specific  distinctness. 
Darwin  showed  clearly  that  this  belief,  al- 
though very  generally  true,  is  by  no  means 
universally  so;  and  his  work  on  heterostyled 
plants  showed  that  all  the  phenomena  of  hy- 
bridism were  displayed  among  forms  that 
certainly  belonged  to  the  same  species.  He 
triumphantly  overthrew  the  doctrine  that 

1  Different  Forms  of  Flowers  on  Plants  of  the  same 
Species,  pp.  242,  243. 


ANALOGY.  105 

mutual  sterility  is  a  mark  of  specific  distinct- 
ness. But  Huxley,  in  his  essay  on  the  "Com- 
ing of  Age  of  the  '  Origin  of  Species,'  "  said: 
"  In  my  earliest  criticisms  of  the  '  Origin'  I 
ventured  to  point  out  that  its  logical  founda- 
tion was  insecure  so  long  as  experiments  in 
selective  breeding  had  not  produced  varieties 
which  were  more  or  less  infertile;  and  that 
insecurity  remains  up  to  the  present  time."1 
Such  was  the  serious  nature  of  the  facts  of 
hybridism  which  needed  explanation.  And  it 
was  the  close  parallelism  between  hybridism 
and  heterostylism  that  led  Darwin  to  seek  in 
the  latter  an  explanation  of  the  difficulties  pre- 
sented by  the  former.  From  the  study  of  the 
illegitimate  offspring  of  heterostyled  plants  he 
drew  the  conclusion  that  the  sterility  is  due, 
not  to  structural  differences,  but  to  functional 
differences  between  the  sexual  elements;  and 
that  it  is  not  due  directly  to  natural  selection, 
but  is  an  incidental  result  accompanying  the 
adaptation  of  the  sexual  elements  of  the  dif- 
ferent forms  of  plants  of  the  same  species  to 
fertilize  each  other.  By  inverting  the  analogy 
he  transferred  the  conclusion  to  the  facts  of 
hybridism.  He  said  that  it  was  this  consider- 
ation, that  the  sterility  of  species  when  first 

1  Life  and  Letters,  Vol.  I.  p.  551. 


106  THE  METHOD   OF  DARWIN. 

crossed,  and  of  their  hybrid  offspring,  is  due 
to  functional  differences  of  the  sexual  elements, 
and  not  to  structural  differences  between  the 
species,  that  led  him  to  make  the  many  experi- 
ments on  the  illegitimate  offspring  of  hetero- 
styled  plants,  and  that  made  the  results  worthy 
of  publication.  The  great  strength  of  the 
analogy  in  his  mind  was  doubtless  due  to  the 
fact  that  the  parallelism  was  so  very  close  as  to 
force  the  conclusion  that  the  two  sets  of  results, 
one  within  a  species,  the  other  between  species, 
are  due  to  the  same  cause. 


VIII. 

INDUCTION. 

A  T  7HEN  Darwin  had  once  grasped  the  idea 
*  *  of  the  descent  of  species,  and  natural 
selection  as  the  cause  determining  modifica- 
tion, it  was  inevitable  that  he  should  look  upon 
all  classes  of  biological  facts  as  consequences 
of  these.  Accordingly,  nearly  all  the  investi- 
gations which  he  carried  on  were  pursued  as 
deductions  from  his  general  principles.  But 
although  the  larger  outlines  and  a  large  part 
of  the  details  of  his  work  were  deductive,  he 
was  frequently  obliged  to  pass  by  induction 
from  facts  to  the  subordinate  principles  which 
he  established 

One  of  the  earliest  of  the  many  instances  in 
which  he  felt  compelled  to  re-interpret  whole 
groups  of  facts  was  that  relating  to  human  ex- 
pression. When,  in  1838,  he  read  Sir  Charles 
Bell's  great  work  on  the  "Anatomy  of  Expres- 
sion," the  view  of  the  latter  that  man  had  been 
created  with  certain  muscles  especially  adapted 
for  the  expression  of  his  feelings  struck  him 


108  THE  METHOD    OF  DARWIN. 

as  unsatisfactory.  As  a  deduction  from  his 
general  theory,  he  believed  that  the  habit  of 
expressing  our  feelings  by  certain  movements 
had  been  somehow  gradually  acquired.  This 
view  required  that  the  whole  subject  of  expres- 
sion should  be  studied  under  a  new  aspect,  and 
each  expression  be  given  a  rational  explana- 
tion.1 This  led  him  to  undertake  his  work  on 
the  "  Expression  of  the  Emotions  in  Man  and 
the  Lower  Animals."  Deduction  pointed  out 
that  expression  must  fall  under  the  general 
explanation;  but  it  was  impossible  to  foresee 
the  principles  which  governed  the  develop- 
ment of  the  various  expressions.  From  1838 
to  1872  he  toiled  away  at  the  mass  of  complex 
facts,  and  slowly  overcame  the  difficulty  of 
bringing  the  different  expressions  together 
under  one  or  a  few  points  of  view.  At  last'  he 
was  able  to  educe  three  principles  of  expres- 
sion, which  seemed  to  him  to  account  for  most 
of  the  expressions  and  gestures  involuntarily 
used  by  man  and  animals  under  the  influence 
of  various  emotions  and  sensations.  These 
principles  he  arrived  at,  as  he  himself  says, 
only  at  the  close  of  his  observations.2  They 

1  Expression   of   the    Emotions   in   Man    and   the   Lower 
Animals,  p.  19. 

2  Ibid.,  p.  27. 


INDUCTION.  IC>9 

are:  (i)  the  principle  of  serviceable  asso- 
ciated habits;  (2)  the  principle  of  antithesis; 
(3)  the  principle  of  actions  due  to  the  constitu- 
tion of  the  nervous  system,  independently  from 
the  first  of  the  will,  and  independently  to  a 
certain  extent  of  habit.  It  is  to  be  remarked 
of  these  three  principles  that  they  are  induc- 
tions, and  that  they  are  vague  on  two  accounts: 
they  are  in  part  so  general  that  it  might  prove 
difficult  to  bring  them  to  a  crucial  test  with 
the  hope  of  proving  or  disproving  them;  and 
the  only  test  to  which  they  have  been  put  is 
that  of  explaining  the  very  facts  from  which 
they  were  drawn.  They  have  not  been  used  to 
make  further  discoveries;  they  represent  well 
the  type  of  inductions  based  on  many  carefully 
studied  facts,  but  unsupported  by  a  subsequent 
deductive  research. 

A  simpler  case  of  induction  is  his  inference 
concerning  the  age  at  which  characters  appear 
which  are  inherited  by  one  or  both  sexes.  "  It 
is  in  itself  probable,"  he  said,  "that  any  char- 
acter appearing  at  an  early  age  would  tend  to 
be  inherited  equally  by  both  sexes,  for  the 
sexes  do  not  differ  much  in  constitution,  before 
the  power  of  reproduction  is  gained";  and 
went  on  to  point  out  that  characters  appearing 
late  in  one  sex  would  tend  to  be  restricted  to 


110  THE  METHOD   OF  DARWIN. 

that  sex.  "  I  was  led  to  infer  that  a  relation  of 
this  kind  exists  from  the  fact  that  whenever 
and  in  whatever  manner  the  adult  male  has 
come  to  differ  from  the  adult  female,  he  differs 
in  the  same  manner  from  the  young  of  both 
sexes."  1 

The  principle  that  characters  appearing  at  an 
early  age  are  inherited  by  both  sexes,  and  char- 
acters appearing  late  in  one  sex  are  restricted 
to  that  sex,  is  an  induction  from  certain  strik- 
ing differences  between  the  adult  males  and 
females,  and  between  the  adult  male  and  the 
young  of  both  sexes  in  many  species.  By 
itself,  without  verification  or  other  deductive 
bracing,  it  would  have  been  an  interesting  gen- 
eralization. But  Darwin  sought  to  strengthen 
it  both  by  observing  whether  it  held  true  in 
particular  cases,  and  by  deducing  it  from  more 
general  laws.  In  the  first  of  the  two  quotations 
given  above,  he  pointed  out  the  probability 
that  the  truth  of  the  principle  depends  on  the 
known  changes  that  take  place  in  the  constitu- 
tion of  the  sexes  on  approaching  maturity.  It 
occurred  to  him  to  put  the  principle  to  a  crucial 
test,  and  to  rely  on  the  result.  Thereupon  fol- 
lowed the  investigation  on  the  deer  family, 
described  elsewhere  in  more  detail. 

1  Descent  cf  Man,  etc.,  p.  276. 


INDUCTION.  Ill 

Inductions  are  easily  made;  the  test  of  a 
good  investigator,  however,  is  not  the  number 
of  inductions  he  makes,  but  his  subsequent 
treatment  of  them.  In  his  study  of  the  amount 
of  material  brought  to  the  surface  of  the  ground 
by  earthworms,  Darwin  noticed  that  the  sur- 
faces of  old  worm-castings  were  often  studded 
with  coarse  particles;  and  was  thence  led  to 
infer  that  a  good  deal  of  the  finest  part  of 
worm-castings  was  washed  away  by  the  rain. 
Such  an  induction  is  so  nearly  self-evident  that 
it  would  seem  superfluous  to  verify  it.  Darwin 
.thought  otherwise;  he  mixed  fine  precipitated 
chalk  with  the  castings,  or  gently  stuck  it  on 
to  them ;  the  rain  washed  it  away  and  proved 
him  correct.  By  this  little  induction,  and  a 
verification  almost  childish  in  its  simplicity 
and  apparent  insignificance,  he  was  able  to  show 
that  the  amount  of  material  brought  to  the 
surface  by  earthworms  is  much  greater  than  at 
first  appears.1  It  is  probably  safe  to  say  that 
the  majority  even  of  investigators  would  have 
regarded  the  induction  sufficient  unto  itself, 
and  would  not  have  hesitated  to  use  it  without 
verification  as  evidence  in  proof  of  their  views. 
We  here  get  a  glimpse  of  how  sharply  Darwin 
caught  the  significance  of  the  minutest  indica- 

1  The  Formation  of  Vegetable  Mould,  etc.,  p.  272. 


112  THE   METHOD   OF  DARWIN. 

tions,  and  of  the  patience  with  which  he  made 
experiments  to  prove  things  which  to  others 
would  seem  so  simple  and  self-evident  as  to 
need  no  proof.  Like  his  great  contemporary, 
Faraday,  he  "  could  trust  a  fact,  and  always 
cross-examined  an  assertion."  It  was  his  con- 
scientious verification  of  even  his  smallest 
inductions  that  gave  the  scientific  world  its 
great  confidence  in  his  work. 

Darwin's  greatest  induction  has  yet  to  be  con- 
sidered, and  will  be  discussed  at  some  length ; 
because,  as  well  as  being  his  greatest  induc- 
tion, it  is  his  most  notable  speculative  failure, 
and  will  give  an  opportunity  to  study  the  char- 
acteristics of  false  and  true  hypotheses.  The 
problem  of  inheritance,  the  transmission  of 
qualities  from  parent  to  offspring,  had  weighed 
upon  him  during  all  the  years  of  his  work  on 
the  theories  of  descent  and  natural  selection. 
Almost  at  the  very  start  we  find  him  making 
experiments  on  beds  of  self-  and  cross-fertilized 
plants  to  determine  questions  of  inheritance. 
These  experiments  possess  their  greatest  inter- 
est, not  from  having  furnished  any  important 
data  for  the  solution  of  the  problem  of  inherit- 
ance, but  from  having  finally  impressed  him 
with  the  importance  of  cross-fertilization  in 
the  plant  kingdom.  They  show  how  early  he 


INDUCTION.  113 

appreciated  the  connection  between  inheritance 
and  his  general  theories,  and  tried  to  get  an 
experimental  basis  for  inference. 

Darwin,  as  he  himself  confessed,  had  to  make 
a  theory  on  every  subject,  and  the  intimate 
relation  between  inheritance  and  his  other 
theories  led  him  irresistibly  to  form  a  theory 
on  inheritance.  Like  Newton,  he  established 
the  best  of  theories,  and,  like  him,  "was  also 
capable  of  proposing  one  of  the  worst."  He 
finally  published  the  hypothesis  under  the 
title  uPangenesis,"  in  Volume  II.  of  "Varia- 
tion of  Animals  and  Plants  under  Domestica- 
tion." 1  The  reason  he  gave  for  forming  it  was 
to  bring  the  vast  number  of  perplexing  facts  of 
inheritance  together  under  a  single  intelligible 
point  of  view.  To  do  this  he  assumed  the 
existence  of  minute  bodies  called  gemmules, 
which  are  cast  off  by  all  the  living  cells  of  the 
animal  or  plant  body  at  all  stages  of  its  exist- 
ence, and  which  multiply  by  division,  have  the 
power  of  remaining  dormant  through  an  in- 
definite number  of  generations,  possess  certain 
remarkable  affinities,  etc.  They  were  sup- 
posed to  collect  in  the  reproductive  elements, 
and  determine  the  character  of  the  offspring. 
Darwin  endeavored  to  make  the  assumption 

i  Pages  349-399- 
8 


114  THE  METHOD   OF  DARWIN. 

reasonable  by  pointing  out  analogies  between 
the  gemmules  and  the  reproductive  elements, 
between  their  affinities  and  those  of  pollen, 
etc.  ;  and  rebutted  the  objection  of  excessive 
minuteness  by  a  comparison  with  molecules. 
Doubtless  the  gemmules  were  a  development 
of  the  idea  of  reproductive  elements,  or  blood- 
corpuscles,  or  both ;  and  their  peculiar  origin, 
powers,  and  hypothetical  history  were  deter- 
mined by  the  various  facts  that  had  to  be 
explained.  It  was  a  clear  case  of  pure  induc- 
tion, untainted  by  any  direct  or  indirect  evi- 
dence of  the  existence  of  the  gemmules,  or  any 
glimpse  of  the  process  by  which  characters  are 
transmitted.  Darwin  knew  how  speculative 
the  hypothesis  was,  and  justified  it  because  it 
brought  all  the  facts  of  inheritance  together 
under  one  point  of  view. 

In  his  discussion  of  it  he  first  stated  the 
facts  to  be  brought  together,  and  then  the 
hypothesis  with  a  working  explanation  of  it, 
and  finally  tried  to  show,  by  reasoning  deduc- 
tively from  the  hypothesis  to  the  facts  in  which 
it  had  originated,  that  it  explained  them. 

Usually  during  the  effort  to  reach  a  cause  by 
the  road  of  induction  the  cause  itself  is  caught 
sight  of,  either  directly  or  indirectly,  and  it  is 
then  possible  to  formulate  at  once  at  least  a 


INDUCTION:  115 

provisional  hypothesis.  In  the  case  of  Pan- 
genesis  this  was  not  true.  Darwin  gave  the 
following  as  the  principal  reasons  for  believing 
in  natural  selection :  "  (i)  It  is  a  true  or  recog- 
nized cause;  (2)  from  the  analogy  of  change 
under  domestication  by  man's  selection,  and 
(3)  chiefly  from  this  view  connecting  under  an 
intelligible  point  of  view  a  host  of  facts."  To 
this  may  be  added  a  fourth,  which  he  men- 
tioned to  Huxley  in  connection  with  the  third, 
as  being  the  reasons  why  younger  scientists 
would  choose  his  theories  rather  than  the  doc- 
trine of  creation;  namely,  that  it  would  enable 
them  to  search  out  new  lines  of  investigation. 

Let  Pangenesis  be  tried  by  these  four  tests. 
In  the  first  place,  it  was  not  even  claimed  that 
there  was  any  proof,  either  direct  or  indirect, 
of  the  existence  of  the  gemmules.  As  a  cause 
which  had  been  actually  observed,  they  had  no 
existence.  Secondly,  while  analogy  is  often  a 
strong  collateral  argument,  and  was  so  in  the 
case  of  natural  selection,  it  was  a  treacherous 
support  in  the  case  of  Pangenesis.  Perhaps  as 
many  analogies  were  violated,  as  was  pointed 
out  by  Delpino  and  others,  by  the  conception 
of  the  gemmules  as  it  could  muster  to  its  sup- 
port; and  one  of  the  first  essentials  of  an 
argument  from  analogy  is  that  the  points  of 


Il6  THE  METHOD   OF  DARWIN. 

similarity  shall  be  more  numerous  than  the 
points  of  difference.  The  third  and  chief 
reason  which  Darwin  gave  for  belief  in  natural 
selection,  that  it  connected  under  an  intel- 
ligible point  of  view  a  host  of  facts,  is  identi- 
cal with  the  reason  which  he  gave  for  forming 
and  so  tenaciously  clinging  to  the  hypothesis 
of  Pangenesis.  And  this  condition  the  latter 
fulfilled.  It  was  bound  to  do  so  by  the  very 
terms  of  its  origin.  A  hypothesis  was  not 
likely  to  leave  Darwin's  hands  until  it  did 
harmonize  with  the  facts  from  which  it  had 
taken  its  rise. 

The  fourth  reason  for  belief  in  a  theory, 
namely,  that  it  leads  deductively  to  new  inves- 
tigations, and  through  them  to  new  facts,  brings 
up  the  hypothesis  of  Pangenesis  against  a  wall. 
A  cause  may  be  recognized  as  a  working  force"; 
its  claim  to  having  produced  known  effects  may 
be  supported  by  analogy;  and  a  vast  body  of 
phenomena  that  must  be  effects  of  some  cause 
may  be  brought  together  into  harmony  under 
it.  But  it  must  also  be  possible  to  work  out 
other  consequences  of  the  theory. 

Francis  Galton  made  a  determined  effort  to 
test  the  hypothesis.  He  felt  the  pressing 
importance  of  doing  so,  for,  as  he  said,  "Its 
postulates  are  hypothetical  and  large,  so  that 


INDUCTION.  II/ 

few  naturalists  seem  willing  to  grant  them."1 
He  reasoned  that,  if  the  gemmules  or  bearers  of 
inheritable  qualities  existed  in  such  enormous 
numbers  in  the  body,  they  must  be  borne  from 
place  to  place  and  to  the  reproductive  organs 
by  the  blood.  If  this  were  true,  then  by  inject- 
ing blood  from  one  variety  of  rabbit  into  the 
blood-system  of  rabbits  of  another  variety,  the 
gemmules  introduced  with  the  foreign  blood 
would  pass  with  those  proper  to  the  animal 
itself  into  the  repVoductive  elements,  and 
would  modify  the  character  of  the  offspring. 
He  foresaw  the  practical  importance  of  such  a 
result.  Slight  dashes  of  blood  could  be  intro- 
duced by  breeders  to  modify  a  variety;  for 
example,  greyhounds  could  have  a  little  of  the 
bull-dog  instilled  into  them.  At  the  end  of 
his  investigation  he  said,  "  I  have  now  made 
experiments  of  transfusion  and  cross-circulation 
on  a  large  scale  in  rabbits,  and  have  arrived 
at  definite  results,  negativing,  in  my  opinion, 
beyond  all  doubt,  the  truth  of  the  doctrine  of 
Pangenesis."  Thus  ended  the  only  effort  ever 
made  to  test  the  hypothesis  deductively  by 
reasoning  out  its  consequences  and  trying  to 
establish  them  by  experiment. 

1  Proceedings  of  the  Royal  Society,  1871,  Vol  XIX.  p.  394 
et  seq. 


Il8  THE  METHOD   OF  DARWIN. 

It  would  seem  as  if  Galton's  experiments  had 
proved  the  hypothesis  false.  Darwin,  how- 
ever, met  the  criticism,  and  slipped  away  from 
the  results  by  admitting  that  he  would  have 
expected  gemmules  in  the  blood;  but  showed 
that  their  presence  there  was  no  necessary  part 
of  his  hypothesis,  because  the  latter  applied 
to  the  lower  animals  and  to  plants,  which  do 
not  have  blood.1  Darwin's  modest  defence  of 
his  hypothesis  swept  away  not  only  Galton's 
experiments,  but  the  possibility  of  proving  or 
disproving  it.  Lionel  Beale  remarked,  sarcas- 
tically, that  it  might  still  be  possible  to  test  it 
by  cutting  out  a  mass  of  an  animal's  flesh,  and 
grafting  in  its  place  a  piece  from  an  animal  of 
another  variety.2  Darwin's  rejoinder  to  Galton 
was  sound  from  the  former's  point  of  view. 
But  the  hypothesis  was  incapable  of  definite 
proof  or  disproof.  There  was  no  set  of  facts 
left  that  could  be  appealed  to  as  a  test.  A 
good  induction  will  not  only  be  in  harmony 
with  and  bring  under  one  point  of  view  a  host 
of  facts,  but  is  likely  to  be  supported  by  one  or 
more  of  the  following  lines  of  proof:  (i)  inde- 
pendent direct  evidence  of  the  existence  of  the 
cause  involved  in  the  induction ;  (2)  strong  and 

1  Nature,  April  27,  1871,  Vol.  III.  p.  502. 

2  Nature,  May  u,  1871,  Vol.  IV.  p.  25. 


INDUCTION.  119 

independent  analogies;  (3)  the  possibility  of 
deducing  consequences  from  the  hypothesis, 
and  verifying  them  by  observation  or  experi- 
ment; (4)  the  possibility  of  deducing  the  cause 
or  principle  as  an  effect  of  another  still  more 
general. 

Huxley  advised  Darwin  not  to  publish  his 
doctrine  of  Pangenesis ;  but  he  nevertheless  did 
publish  it,  and  gave  as  his  reason  the  pressing 
importance  of  co-ordinating  the  inexplicable 
facts  of  inheritance.  Other  hypotheses  have 
followed  Darwin's  with  as  little  success.  Dar- 
win did  not  formulate  his  hypothesis  to  support 
his  other  theories,  but  its  character  was  at  least 
in  part  determined  by  the  latter.  It  is  interest- 
ing to  note  that  the  latest  hypothesis  of  inherit- 
ance, the  most  aggressive  that  has  yet  arisen, 
has  been  developed  as  a  special  support  for  the 
belief  that  natural  selection  acting  upon  con- 
genital variations  is  the  sole  cause  in  the  pro- 
duction of  species,  and  that  acquired  characters 
are  not  inherited.  The  principal  evidence  on 
which  the  hypothesis  relies  for  support,  apart 
from  the  refutation  of  the  direct  evidence  ad- 
duced to  support  the  belief  in  the  inheritance 
of  acquired  characters,  are  the  karyokinetic 
processes  in  cell  division,  and  the  early  stage 
in  development  at  which,  in  some  animals,  the 


120  THE  METHOD   OF  DARWIN. 

reproductive  elements  become  distinct  from  the 
parts  which  develop  into  the  other  organs  of 
the  body.  In  this  respect  Weissmann's  hypoth- 
esis has  an  additional  logical  support.  It  has 
provoked  earnest  discussion,  and  naturalists 
have  taken  sides  on  the  subject.  Although 
the  germ  plasm  is  itself  an  assumption,  and 
the  hypothesis  owes  its  existence  to  the  known 
facts  of  karyokinesis,  it  has  led  to  further 
investigations  in  some  directions  with  fruitful 
results.  Darwin's  hypothesis  opened  the  ques- 
tion under  the  new  light  that  he  had  shed  upon 
nature,  and  the  more  recent  hypotheses,  like 
his  own,  owe  their  existence  to  the  "  impelling 
force  "  of  his  general  theories  of  descent  and 
natural  selection. 


IX. 


DEDUCTION. -EXPLANATION  OF  KNOWN  FACTS. 

—IMPORTANCE  OF  THEORY   TO   GOOD 

OBSERVATION. 

DARWIN  fell  upon  the  true  cause  of  the 
modification  of  species  so  early,  that  the 
greater  number  of  his  special  investigations 
took  on  a  deductive  cast.1  His  reflections  upon 
the  known  facts  and  principles  of  biology  took 
the  form  of  efforts  to  explain  them  as  deduc- 
tions from  his  theory;  and  many  of  his  new 
discoveries  were  foreseen  as  consequences  of 
it.  Hence  the  inductive  process  does  not  play 
so  important  a  part  in  his  work  as  does  the 
inverse  process  of  deduction. 

It  will  not  be  necessary  to  dwell  long  upon 
his  success  in  giving  the  proper  theoretical  ex- 
planations to  already  known  facts  and  empirical 
laws.  It  had  required  centuries  of  painstaking 
research  and  numberless  efforts  at  classification 
before  anything  like  a  natural  classification  was 

1  See  the  Chapter  on  the  Logical  History  of  the  Principle 
of  Natural  Selection,/^/,  p.  212. 


122  THE  METHOD   OF  DARWIN. 

reached.  When,  at  the  beginning  of  this  cen- 
tury, it  was  finally  approached,  and  the  natural 
affinities  of  plants  and  animals  were  brought 
out  by  it, the  doctrine  of  descent  was  inevitable; 
and  it  came.  When  Darwin  had  once  become 
impressed  with  its  truth,  and  had  found  the 
cause  of  modification,  it  was  first  of  all  neces- 
sary to  show  that  the  great  bodies  of  known 
facts  harmonized  with  his  doctrines.  The  facts 
of  distribution,  palaeontology,  embryology,  rudi- 
mentary organs,  etc.,  were  all  reduced.  Each  set 
of  facts  presented  its  own  difficulties. 

Up  to  the  present  century  it  was  regarded  as 
an  axiom  in  taxonomy  that  the  structures  of 
most  importance  to  the  animals  possessing 
them  must  be  of  most  importance  for  the  pur- 
poses of  classification.  It  is  worth  while  to 
note  that  this  was  accepted  as  self-evident,  as 
being  beyond  the  necessity  of  proof.  Systema- 
tists  were  approaching  the  "natural  arrange- 
ment," and  De  Candolle  discovered  empirically 
the  rule  that  there  is  usually  an  inverse  ra- 
tio between  the  taxonomic  and  the  functional 
value  of  a  structure ;  but  he  could  suggest  no 
reason  for  the  paradox.  Darwin's  theory  fur- 
nished the  philosophical  explanation.1  The 

1  Origin  of  Species,  pp.  362-373.  Romanes,  Darwin  and 
After  Darwin,  pp.  34-37- 


DEDUCTION.  123 

organs  of  the  highest  functional  value  are 
under  the  constant  and  pressing  necessity  of 
changing  with  changes  in  the  environment; 
while  those  of  least  functional  value  remain 
undisturbed,  and  pass,  little  or  not  at  all  modi- 
fied, from  generation  to  generation.  The  same 
explanation  holds  for  the  rule  concerning  the 
importance  of  "aggregates  of  unimportant  char- 
acters "  in  determining  the  affinities  of  animals 
and  plants. 

In  some  parts  of  the  natural  system,  there 
are  what  are  called  "chains  of  affinities."  A 
group,  instead  of  being  broken  up  into  well 
separated  sub-groups,  consists  of  a  chain  in 
which  the  adjacent  parts  are  closely  related, 
but  the  more  distant  parts  have  comparatively 
few  points  in  common.  It  is  impossible  to 
break  up  the  group  without  violating  the  affini- 
ties of  adjacent  parts,  and  it  is  difficult  to  define 
it  in  such  a  way  as  to  include  the  extremes. 
The  Crustacea  furnish  an  example.  What  was 
a  special  difficulty  under  the  old  views  of  clas- 
sification is  explained  under  the  doctrine  of 
descent.1 

One  of  the  most  important  principles  that 
had  been  established  empirically  was  the  tree- 
like arrangement  of  species  and  higher  groups 

1  Origin  of  Species,  p.  368. 


124  THE  METHOD   OF  DARWIN. 

in  the  natural  classification.  The  recognition 
of  the  principle  came  only  after  centuries  of 
efforts  at  classification;  and  after  it  was  dis- 
covered, no  reason  could  be  given  for  it.  Al- 
though so  helpful  and  striking,  it  remained  a 
profound  enigma,  for  there  was  nothing  in  the 
nature  of  the  things  classified  that  required  this 
peculiarly  complex  arrangement  rather  than  one 
of  several  conceivable  simpler  ones.  Darwin's 
explanation  of  this  principle  under  his  theory 
illustrates  not  only  the  explanation  of  empirical 
laws,  but  the  difficulty  of  doing  what,  after  it 
is  done,  seems  very  simple. 

It  was  not  until  after  he  had  been  at  work 
upon  the  principle  of  natural  selection  for 
many  years,  that  the  true  explanation  of  this 
law  under  his  principle  occurred  to  him.  He 
said,  "  I  suppose  I  must  be  a  very  slow  thinker, 
for  you  would  be  surprised  at  the  number  of 
years  it  took  me  to  see  clearly  what  some  of 
the  problems  were  which  had  to  be  solved; 
such  as  the  necessity  of  the  principle  of  diver- 
gence of  character,  the  extinction  of  inter- 
mediate varieties,  on  a  continuous  area,  with 
graduated  conditions,"  etc.1  After  describing 
his  earlier  sketches  of  his  theory  he  said,  "At 
that  time  I  overlooked  one  problem  of  great  im- 

1  Life  and  Letters,  Vol.  I.  pp.  68,  524. 


DEDUCTION.  125 

portance,  ,  .  .  the  tendency  in  organic  beings 
descended  from  the  same  stock  to  diverge  in 
character  as  they  become  modified."  That  they 
have  thus  diverged  was  proved  by  their  arrange- 
ment into  groups  within  groups.  "I  can  re- 
member," he  went  on,  "the  very  spot  in  the 
road,  whilst  in  my  carriage,  when  to  my  joy 
the  solution  occurred  to  me;  and  this  was  long 
after  I  had  come  to  Down."  1  The  solution  was 
this,  — "that  the  modified  offspring  of  all 
dominant  and  increasing  forms  tend  to  become 
adapted  to  many  and  highly  diversified  places 
in  the  economy  of  nature." 

It  seems  very  simple  now  to  understand  that 
the  tree-like  arrangement  of  species  must  be 
regarded  as  a  direct  consequence  of  the  prin- 
ciple of  natural  selection.  It  might  even  seem 
easy  to  infer  the  tree-like  arrangement  as  a  de- 
duction from  the  principle,  and  to  discover  it,  if 
it  had  not  already  been  discovered  empirically. 
It  is  not  because  Darwin  was  so  slow  a  thinker 
that  it  took  him  so  long  to  discover  the  relation 
of  cause  and  effect  between  natural  selection 
and  the  tree-like  arrangement  of  species;  but 
it  was  because  he  was  a  strong  and  persistent 
thinker  that  he  discovered  it  at  all.  The  think- 
ing out  of  such  connections  is  always  a  slow 

1  Life  and  Letters,  Vol.  I.  pp.  68-70. 


126  THE  METHOD   OF  DARWIN. 

process.  The  final  mental  act  may  come  like 
a  flash,  as  it  did  in  Darwin's  case,  and  be  fol- 
lowed by  a  very  rapid  and  fruitful  explanation  of 
details ;  but  such  triumphs  do  not  come  to  the 
mind  that  will  not  serve  the  apprenticeship.1 

It  has  been  often  declared  that  the  work  of 
biologists  since  1860  has  consisted  in  explain- 
ing known  facts  as  deductions  from  Darwin's 
theories,  and  further  investigating  the  conse- 
quences of  those  theories.  The  more  exten- 
sively a  theory  has  been  successfully  applied, 
the  more  easy  it  is  to  do  what  still  remains  to 
be  done.  There  is  profound  philosophy  in  the 
saying,  "To  him  that  hath  shall  be  given."  It 
was  a  comparatively  easy  matter  to  apply  Dar- 
win's theories  to  all  sorts  of  facts  and  lines  of 
investigation  after  he  had  so  thoroughly  tested 
and  illustrated  them ;  but  even  then  the  scien- 
tific world  was  very  long  in  working  out  some 
of  their  striking  consequences.  It  is  not  to  be 
wondered  at  that  Darwin  was  slow  to  under- 
stand many  things,  or  that  he  overlooked  others 
entirely.  It  is  rather  to  be  wondered  at  that 
he  accomplished  so  much  single-handed.  One 

1  My  friend,  Prof.  G.  C.  Price,  has  called  my  attention  to 
the  fact  that  Lincoln  was  marvellously  like  Darwin  in  many 
respects.  The  former  was  noted  for  his  efforts  to  reach  funda- 
mental principles  in  thinking,  and  was  also  noted  for  the 
"  slowness  "  of  his  mental  action. 


OF  THE 


EXPLANATION  OF  KNOWN 

of  the  hardest  and  most  important  lessons  for 
the  majority  of  investigators  to  learn  is  that 
the  chances  are  all  against  their  exhausting 
their  subjects,  or  even  putting  them  into  such 
shape  that  the  work  will  not  all  have  to  be 
done  over  again,  unless  their  work  is  done 
slowly  and  continued  persistently  through  long 
periods  of  time. 

Darwin's  great  logical  power  was  fortified  by 
another  rare  quality  of  mind,  —  unusual  acute- 
ness  in  observing  all  collateral  facts  that  came 
out  in  his  observations  or  experiments,  whether 
they  seemed  to  bear  directly  upon  the  subject 
of  investigation  or  not.  He  was  thus  put  in 
possession  of  many  facts  that  afterwards  proved 
valuable  in  ways  that  he  could  not  foresee. 
But  with  all  his  ability  in  these  directions  he 
experienced  difficulty  in  grasping  the  full 
significance  of  facts. 

In  discussing  Cleistogene  flowers,  near  the 
end  of  his  work  on  the  "  Effects  of  Cross-  and 
Self-Fertilization,"  1  after  giving  the  reasons  for 
the  belief  he  is  about  to  express,  he  said,  "I 
must  believe  that  plants  now  bearing  small  and 
inconspicuous  flowers  profit  by  their  still  re- 
maining open,  so  as  to  be  occasionally  inter- 
crossed by  insects.  It  has  been  one  of  the 

1  Page  387. 


128  THE  METHOD   OF  DARWIN. 

greatest  oversights  of  my  work  that  I  did  not 
experimentize  on  such  flowers,  owing  to  the 
difficulty  of  fertilizing  them  and  to  my  not 
having  seen  the  importance  of  the  subject." 

Although  it  is  clear  that  the  possession  of  a 
theory  is  no  guaranty  that  all  its  consequences 
will  be  foreseen,  or,  if  foreseen,  observed,  or 
even  that,  if  they  are  both  deductively  foreseen 
and  empirically  observed,  they  will  be  brought 
into  connection  with  the  theory;  nevertheless, 
the  importance  of  theory  for  accurate  observa- 
tion cannot  be  overestimated.  However  cau- 
tious Darwin  was  about  committing  himself 
unreservedly  to  a  hypothesis,  he  never  really 
dispensed  with  one  if  he  could  find  one. 
Though  he  subdued  his  tendency  to  speculation 
in  the  interest  of  observation,  he  did  not  dis- 
pense with  at  least  provisional  hypotheses,  even 
in  accumulating  his  facts.  He  felt  the  want 
when  he  could  not  find  one,  and  made  it  his 
first  task  to  establish  some  degree  of  probability 
in  favor  of  one. 

One  of  his  early  experiences  is  a  good  illus- 
tration of  how  even  trained  observers  could  not, 
without  the  help  of  a  theory,  observe  phenom- 
ena on  which  they  actually  walked,  and  which 
obstructed  their  progress.1  He  has  said,  "I 

1  Life  and  Letters,  Vol.  I.  pp.  48,  49. 


IMPORTANCE  OF  THEORY.  I2Q 

had  a  striking  instance  of  how  easy  it  is  to 
overlook  phenomena,  however  conspicuous,  be- 
fore they  have  been  observed  by  any  one.  We 
spent  many  hours  at  Cwm  Idwal,  examining 
the  rocks  with  extreme  care,  as  Sedgwick  was 
anxious  to  find  fossils  in  them,  but  neither  of 
us  saw  a  trace  of  the  wonderful  glacial  phe- 
nomena all  around  us;  we  did  not  notice  the 
plainly  scored  rocks,  the  perched  boulders,  the 
lateral  and  terminal  moraines,  yet  these  phe- 
nomena are  so  conspicuous  that,  as  I  declared 
in  a  paper  published  many  years  afterwards  in 
the  Philosophical  Magazine  (1842),  a  house 
burnt  down  by  fire  did  not  tell  its  story  more 
plainly  than  did  this  valley.  If  it  had  been 
filled  with  a  glacier,  the  phenomena  would  have 
been  less  distinct  than  they  now  are." 

It  may  seem  strange  that  two  men,  one  already 
a  famous  geologist  and  the  other  soon  to  be- 
come one,  should  have  overlooked  such  evi- 
dence, which  has  since  become  so  interesting, 
so  widely  known,  and  so  well  understood.  The 
secret  of  their  failure  is  that  they  were  not 
looking  for  it.  It  is  usually  the  things  that 
men  look  for  that  they  see;  and  to  look  for 
things  as  yet  unseen  requires  a  theory  as  a 
headlight.  Even  if  they  had  noticed  the  mate- 
rial of  the  moraines,  the  perched  boulders,  and 
9 


130  THE  METHOD    OF  DARWIN. 

the  parallel  scratches,  they  would  certainly  not 
have  been  deeply  impressed  by  them,  because 
they  become  impressive  only  when  their  rela- 
tion to  one  another  is  understood,  and  this 
could  only  be  when  the  glacial  theory  had 
been  imported  from  a  glacier  country. 

The  importance  of  the  discovery  of  the  theory 
of  natural  selection  to  the  work  of  Darwin's  life 
will  be  dwelt  upon  later.  A  sigh  of  relief  is 
embodied  in  the  declaration,  "  Here,  then,  I 
had  at  last  got  a  theory  by  which  to  work."1 
Facts  cannot  be  seen  without  some  notion  of 
the  relation  they  will  bear  to  each  other  when 
they  are  found.  The  stupendous  importance 
of  theory  for  observation  is  illustrated  by  the 
effect  of  Darwin's  theories  on  biological  inves- 
tigation in  all  its  phases.  Huxley  put  it  thus : 
"The  '  Origin'  provided  us  with  the  working 
hypothesis  we  sought."2  The  whole  biological 
world  was  waiting  for  it;  and  when  it  came  it 
carried  the  biological  sciences  into  the  deduc- 
tive stage,  and  opened  an  era  of  investigation 
unprecedented  in  the  rapidity  with  which  dis- 
covery advanced,  and  in  the  accuracy  of  the 
results  reached. 

There    are    scattered    throughout    Darwin's 

1  Life  and  Letters,  Vol.  I.  p.  68. 

2  Ibid.,  p.  551. 


IMPORTANCE   OF  THEORY,  131 

works  numerous  illustrations  of  the  importance 
of  theory  in  the  investigation  even  of  matters 
of  detail.  Writing  of  the  trimorphic  Lythrum 
salicaria,  he  said,  "The  existence  of  the  three 
forms  was  first  observed  by  Vaucher,  and  sub- 
sequently by  Wirtgen ;  but  these  botanists,  not 
being  guided  by  any  theory  or  even  suspicion 
of  their  functional  differences,  did  not  perceive 
some  of  the  most  curious  points  of  difference 
in  their  structure."1  MM.  Boitard  and  Corbie, 
in  their  study  of  pigeons,  had  seen  and  recorded 
many  facts  which  they  could  not  use,  simply 
from  lack  of  a  theory.  They  had  stated  that 
when  they  crossed  certain  breeds  of  pigeons, 
birds  colored  like  the  Columba  livia,  or  the 
common  dove-cot,  were  almost  invariably  pro- 
duced.2 Darwin  gave  significance  to  these  facts 
and  many  others  by  the  theory  of  descent. 

In  spite  of  his  unusual  power  of  seeing  facts 
apparently  unconnected  with  the  subject  under 
investigation,  and  his  persistent  habit  of  record- 
ing results,  whatever  they  might  be,  Darwin 
himself,  sometimes  "  not  foreseeing  the  result, 
did  not  keep  a  memorandum  of  all  the  facts," 
which  would  afterwards  have  proved  useful. 

1  Different    Forms   of    Flowers   on    Plants   of    the    same 
Species,  p.  138. 

2  Variation  of  Animals  and   Plants   under   Domestication, 
Vol.  II.  p.  14. 


1 3  2  THE  ME  THOD   OF  DAR  WIN. 

No  one  was  more  thoroughly  convinced  of  the 
necessity  of  clear-cut  theory  for  accurate  obser- 
vation; and  he  frequently  expressed  himself  to 
that  effect.  His  son  says  of  him,  "He  often 
said  that  no  one  could  be  a  good  observer  unless 
he  was  an  active  theorizer. "  1  He  said  him- 
self, "  I  am  a  firm  believer  that  without  specula- 
tion there  is  no  good  and  original  observation."  2 
"  It  is  an  old  and  firm  conviction  of  mine  that 
the  naturalists  who  accumulate  facts  and  make 
many  partial  generalizations  are  the  real  bene- 
factors of  science.  Those  who  merely  accumu- 
late facts  I  cannot  very  much  respect."  3 

1  Life  and  Letters,  Vol.  I.  p.  126. 

2  Ibid.,  p.  465. 

8  Ibid.,  Vol.  II.  p.  21. 


X. 

DEDUCTION.  —  ANTICIPATION. 

A  VERY  few  negative  instances  have  been 
•**•  given  to  show  the  importance  of  theory 
for  accurate  observation.  They  illustrate  how 
the  absence  of  theory  led  to  the  oversight  or 
neglect  of  facts  which  later,  under  the  sway 
of  theory,  have  become  important.  Darwin's 
works  are  full  of  instances  in  which  he  was  led 
by  his  theory  to  anticipate  the  facts  of  nature. 
It  was  inevitable  that,  having  so  early  dis- 
covered the  theories  which  covered  the  whole 
territory  in  which  he  worked,  he  should  be 
guided  by  them  in  the  search  for  facts,  and 
that  his  work  should  thenceforth  be  deductive 
in  its  character.  Examples  of  this  character- 
istic method  range  from  the  great  deductions 
which  led  to  nearly  all  his  important  special 
investigations,  and  which  illustrate  the  sweep- 
ing consequences  of  his  general  theories,  to  the 
little  deductive  details  which  show  how  swift 
and  accurate  his  prevision  became  even  in  the 
matter  of  minute  consequences  of  these  theories. 


134  THE  METHOD   OF  DARWIN. 

The  minor  instances  will  be  given  first,  and 
will  be  followed  by  the  more  general  ones. 
Then  will  follow  deductions  which  he  made, 
but  which  are  still  unverified,  or  have  been 
verified  by  others;  and  lastly  will  be  given 
some  of  the  instances  in  which  he  went  clearly 
wrong  in  his  deductions. 

The  instance  about  to  be  given  may  well  be 
placed  first,  for  the  purpose  of  raising  a  mooted 
question  in  logic.  Mill  took  the  position  that 
typically  the  process  of  inference  consists  in 
reasoning  directly  from  one  particular  case  to 
another;  whereas  the  older  and  more  generally 
accepted  view  is  that  inference  must  pass  by 
induction  from  particulars  to  a  general  law,  and 
then  by  deduction  from  the  general  law  to  other 
particulars.  Darwin  had  found  silicified  wood 
in  certain  tufaceous  formations  in  Patagonia 
and  on  the  island  of  Chiloe  on  the  west  coast 
of  South  America.  He  afterwards  crossed  the 
Andean  Cordillera  in  an  east  and  west  direc- 
tion, and  again  found  tufaceous  formations. 
In  his  description  of  the  geological  section  of 
the  Uspallata  range  he  said:  "Many  of  these 
tufaceous  beds  resemble,  with  the  exception  of 
being  more  indurated,  the  upper  beds  of  the 
great  Patagonian  Tertiary  formation,  especially 
those  variously  colored  layers  high  up  the  river 


DED  UCTION.  —  ANTICIPA  TION.  1 3  5 

Santa  Cruz,  and  in  a  remarkable  degree  the 
tufaceous  formations  at  the  northern  end  of 
Chiloe.  I  was  so  much  struck  with  this  re- 
semblance, that  I  particularly  looked  out  for 
silicified  wood,  and  found  it  under  the  follow- 
ing extraordinary  circumstances."  1 

We  are  not  told  whether  he  gained  his  first 
knowledge  of  the  connection  between  silicified 
wood  and  tufaceous  formations  from  his  geo- 
logical researches  in  South  America;  nor  if  he 
did,  whether  he  had  come  to  the  general  con- 
clusion that  silicified  wood  is  likely  to  be  found 
in  tufaceous  formations,  and  therefore  it  ought 
to  be  found  in  the  beds  of  the  Uspallata  range, 
or  whether  he  simply  reasoned  that  because 
silicified  wood  is  found  in  the  tufaceous  beds  of 
Chiloe,  therefore  it  will  be  found  in  the  very 
similar  tufaceous  beds  of  the  Uspallata  range. 
It  is  known  that  the  two  are  found  in  connec- 
tion in  different  parts  of  the  world,  and  there 
is  a  causal  connection  between  them.  The 
peculiar  conditions  under  which  tufaceous  beds 
are  deposited,  and  the  subsequent  processes  of 
mineral  solution,  etc.,  are  such  that,  if  wood 
was  present  at  the  time  of  the  deposition 
of  such  beds,  it  would  probably  be  silicified. 
Whether  Darwin  knew  of  this  causal  connec- 

1  Geological  Observations,  etc.,  p.  526. 


136  THE  METHOD  OF  DARWIN. 

tion  or  only  knew  empirically  that  the  two  are 
often  found  in  connection,  if  he  reasoned  from 
a  belief  that  the  two  are  likely  to  be  found 
together,  the  Uspallata  case  is  one  of  deduc- 
tion. It  would  be  interesting  to  have  a  com- 
plete record  of  the  mental  operations  in  this 
case;  but  from  Darwin's  mental  habit  of  leap- 
ing quickly  to  hypotheses,  even  where  the 
connection  between  facts  appeared  altogether 
empirical,  there  cannot  be  much  doubt  about 
the  deductive  character  of  the  process,  even 
if  it  be  held  that  the  process  of  inference  is 
typically  from  one  particular  case  to  another. 

In  his  studies  of  coral  islands  he  laboriously 
gathered  information  concerning  the  distribu- 
tion of  atolls,  barrier  reefs,  and  fringing  reefs, 
and  indicated  each  kind  by  a  different  color  on 
the  map,  distinguishing  fringing  reefs  with  red. 
He  had  reached  the  conclusion  that  atolls  and 
barrier  reefs  lie  in  areas  of  subsidence,  and 
fringing  reefs  in  areas  of  elevation.  So  far  as 
he  could  learn  from  records,  Banks  Islands  and 
some  others  apparently  had  no  fringing  reefs, 
although  they  lie  in  what  he  had  designated 
the  zone  of  elevation.  He  pointed  out  the 
fact  that  most  of  the  information  concerning 
coral  reefs  and  islands  had  been  collected  and 
recorded  in  the  interests  of  navigation,  and 


DED  UC  TION.  —  ANTTCIPA  TION.  I  3  / 

from  this  point  of  view  narrow  fringing  reefs 
would  be  insignificant,  and  likely  to  be  over- 
looked. He  said,  "  I  do  not  doubt  that  several 
of  these  islands,  now  left  uncolored  [on  his 
map],  ought  to  be  red."  Bonney,  in  his  revised 
edition  of  Darwin's  "Coral  Reefs,"  says,  on 
the  authority  of  Captain  Wharton,  that  Banks 
Islands  are  fringed  in  parts.1 

The  data  from  which  he  had  compiled  the 
map  of  the  distribution  of  coral  reefs  and 
islands  had  been  recorded  for  a  purpose  entirely 
different  from  that  for  which  he  wished  to  use 
them ;  hence  much  information  that  was  impor- 
tant for  him  was  left  out.  The  imperfect  data 
verified  his  principle  of  distribution;  and  he 
was  able  to  infer  deductively  that  there  were 
small  fringing  reefs  in  existence  where  they 
had  not  been  observed  or  recorded. 

In  pursuit  of  the  same  subject  he  studied 
carefully  the  charts  of  the  Great  Chagos  Bank, 
which  he  had  not  seen  himself.  He  saw  from 
a  study  of  the  chart  that,  as  he  said,  "  On  the 
eastern  side  of  the  atoll  some  of  the  banks  are 
linear  and  parallel,  like  islets  in  a  great  river, 
and  they  pointed  directly  towards  a  great  breach 
on  the  opposite  side  of  the  atoll.  I  inferred 
from  this  that  strong  currents  sometimes  set 

1  Structure  and  Distribution  of  Cpral  Reefs,  p.  220. 


138  THE  METHOD   OF  DARWIN. 

directly  across  this  great  bank;  and  I  hear 
from  Captain  Moresby  that  this  is  the  case."1 
The  causal  relation  between  currents  of  water 
and  linear  parallel  islets  was  known  to  him; 
from  this  law  and  the  particular  arrangement 
of  the  banks  he  deduced  the  existence  of  cur- 
rents, and  verified  the  anticipation. 

Some  of  these  instances  are  so  simple  as 
almost  to  require  apology  for  their  insertion ; 
but  their  very  simplicity  makes  them  typical 
of  the  most  common  of  the  logical  processes. 
The  variety  of  consequences  to  be  deduced 
from  principles  or  laws  is  so  great,  that  induc- 
tions by  which  those  laws  or  principles  are 
reached  are  few  compared  with  the  deductions 
by  which  their  consequences  are  developed. 

Darwin's  studies  of  the  fertilization  of  flowers 
are  full  of  examples  of  deduction  and  verifica- 
tion ;  and  some  of  these  are  very  curious.  The 
principle  of  advantage  from  cross-fertilization 
lay  at  the  root  of  these  deductions;  and  it  was 
by  incessant  application  of  it  that  he  was  able 
to  interpret  the  most  complex  arrangements  in 
orchidaceous  flowers.  He  studied  the  structure 
of  Listeraovata,  and  experimented  on  the  action 
of  its  parts,  until  he  felt  sure  that  he  under- 
stood the  manner  in  which  insects  enter  the 

1    Structure  and  Distribution  of  Coral  Reefs,  p.  150. 


DED  UC  TIO  N.  —  A  NTICIPA  TION.  1 3  9 

flower  and  free  the  pollinia.  After  watching 
flowers  for  hours  to  see  Nature  at  work,  he  was 
rewarded  with  a  verification  of  his  interpreta- 
tion.1 In  the  description  of  another  case  he 
said,  "  From  the  large  size  of  the  flower,  more 
especially  of  the  viscid  disk,  and  from  its  won- 
derful power  of  adhesion,  I  formerly  inferred 
that  the  flowers  were  visited  by  large  insects, 
and  this  is  now  known  to  be  the  case."  2  He 
was  well  acquainted  with  the  general  interaction 
of  the  parts  and  their  relation  to  visiting  insects. 
From  this  and  the  size,  etc.  of  the  parts,  he 
was  able  to  deduce  the  size  and  strength  of  the 
insect.  He  had  been  convinced  by  theoretical 
considerations  that  the  pollen  of  Hedychium  is 
removed  by  the  wings  of  hovering  butterflies, 
and  wrote  to  India  to  have  the  butterflies  ob- 
served in  action.  Two  years  later  Fritz  Miiller 
observed  the  process  itself. 

Probably  no  other  of  Darwin's  works  illus- 
trates so  well  the  variety  of  results  which  may 
be  deduced  from  a  general  principle  by  inge- 
nuity and  skill  in  interpretation  as  does  that  on 
the  " Fertilization  of  Orchids."  When  once 
he  had  laid  down  the  proposition  that  the 
flowers  of  many  plants  are  adapted  for  cross- 

1  Fertilization  of  Orchids,  pp.  119,  120. 

2  Ibid.,  p.  190. 


140  THE  METHOD   OF  DARWIN, 

fertilization,  he  set  himself  the  task  of  inter- 
preting the  complex  arrangement  of  parts  in 
orchidaceous  flowers  in  relation  to  the  visiting 
insects  in  accordance  with  that  proposition. 

One  of  the  most  interesting  and  successful 
applications  of  this  principle  is  the  following. 
He  has  described  various  excrescences,  warts, 
ribs,  ridges,  etc.  on  the  labellum,  or  large  odd 
petal  of  different  orchids,  the  flowers  of  which 
do  not  secrete  nectar.  He  was  haunted  by 
the  question  of  the  meaning  of  these  absurdly 
insignificant  and  irregular  things,  — their  rela- 
tion to  the  other  parts  and  to  the  visiting  in- 
sects. He  said,  "  From  the  position  relatively 
to  the  viscid  disk  [by  means  of  which  the  pol- 
len is  removed  by  visiting  insects]  which  these 
excrescences  occupy,  and  from  the  absence  of 
any  free  nectar,  it  formerly  seemed  to  me  higlrly 
probable  that  they  afforded  food,  and  thus 
attracted  either  Hymenoptera  or  flower-feeding 
Coleoptera.  .  .  .  Nevertheless  it  was  a  bold 
speculation  that  insects  were  attracted  to  the 
flowers  of  various  orchids  in  order  to  gnaw  the 
excrescences  or  other  parts  of  the  labella;  and 
few  things  have  given  me  more  satisfaction 
than  the  full  confirmation  of  this  view  by  Dr. 
Cruger,  who  has  repeatedly  witnessed  in  the 
West  Indies  humble-bees  of  the  genus  Euglossa 


DED  UC  TION.  —  A  NTICIPA  TION.  1 4 1 

gnawing  the  labellum  of  Catasetum,  CoryantJius, 
G angora,  and  StanJiopea. "  1 

When  his  attention  was  first  attracted  to  the 
cowslip  (Primula  veris),  and  he  noticed  the 
different  forms  of  flowers  on  different  plants, 
some  with  longer  pistils,  rougher  stigmas,  and 
smaller  pollen  grains,  and  some  with  shorter 
pistils,  longer  stamens,  and  larger  pollen  grains, 
he  inferred  that  the  species  was  tending  to 
become  dioecious.  Later  he  became  convinced 
that  the  differences  between  the  forms  were  for 
the  purpose  of  securing  cross-fertilization,  and 
proved  this  to  be  the  case.  In  his  discussion 
of  the  positions  of  the  flower  organs  in  the  long- 
styled  and  short- styled  forms  he  said:  "The 
anthers  in  the  one  form  stand  nearly,  but  not 
exactly,  on  a  level  with  'the  stigma  of  the 
other.  ...  It  follows  from  the  position  of  the 
organs  that  if  the  proboscis  of  a  dead  humble- 
bee,  or  a  thick  bristle,  or  a  rough  needle  be 
pushed  down  the  corolla,  first  of  one  form  and 
then  of  the  other,  as  an  insect  would  do  in  visit- 
ing the  two  forms  growing  mingled  together, 
pollen  from  the  long-stamened  form  adheres 
around  the  base  of  the  object,  and  is  left  with 
certainty  on  the  stigma  of  the  long-styled  form  ; 
whilst  pollen  from  the  short  stamens  of  the 

1  Fertilization  of  Orchids,  pp.  269,  270. 


142  THE  METHOD  OF  DARWIN. 

long-styled  form  adheres  a  little  way  above  the* 
extremity  of  the  object,  and  some  is  generally 
left  on  the  stigma  of  the  other  form.  In  ac- 
cordance with  this  observation  I  found  that 
the  two  kinds  of  pollen,  which  could  easily  be 
recognized  under  the  microscope,  adhered  in 
this  manner  to  the  proboscides  of  the  two 
species  of  humble-bees  and  of  the  moth  which 
were  caught  visiting  the  flowers."  l 

From  these  apparently  insignificant  deduc- 
tions and  verifications  he  passed  on  to  show 
that  the  short-styled  form  was  far  more  likely 
to  be  self-fertilized  than  the  long-styled  form. 
For  when  he  inserted  a  bristle  or  other  object 
into  the  corolla  of  the  short-styled  form,  he 
had  to  pass  it  between  the  anthers  seated  round 
the  mouth  of  the  corolla,  and  some  of  the 
pollen  was  almost  invariably  carried  down  arid 
left  on  the  stigma.  His  inferences  concerning 
adaptation  for  cross-fertilization  in  these  forms 
were  completely  verified,  so  far  as  it  could  be 
done  from  a  study  of  the  relative  positions  of 
the  parts  of  the  flowers  and  the  action  of  insects 
upon  them.  The  proof  was  strong  enough  to 
support  the  principle.  But  for  Darwin  it  was 
merely  made  the  starting  point  for  further 
verification  from  the  physiological  side.  "The 

1  Different  Forms  of  Flowers,  etc.,  pp.  18-24. 


DED  UCTION'.  —  ANTICIPA  TION.  1 43 

several  foregoing  facts  led  me  to  try  the  effects 
of  the  two  kinds  of  pollen  on  the  stigmas  of 
the  two  forms."  This  "trial"  consisted  of  a 
long  series  of  experiments  to  establish  the 
effects  of  legitimate  and  illegitimate  unions  of 
the  two  forms.  The  reward  of  this  effort  to 
establish  the  principle  by  physiological  evi- 
dence is  summed  up  in  his  own  words:  "From 
the  facts  now  given  the  superiority  of  a  legiti- 
mate over  an  illegitimate  union  admits  of  not 
the  least  doubt ;  and  we  have  here  a  case  to 
which  no  parallel  exists  in  the  vegetable,  or 
indeed  in  the  animal  kingdom."1 

It  had  long  been  known  that  if  pollen  from 
a  distinct  species  is  placed  on  the  stigma  of  a 
plant,  and  afterwards  (sometimes  many  hours 
afterwards)  pollen  from  its  own  species  is 
placed  on  the  same  stigma,  the  latter  obliter- 
ates the  effects  of  the  former,  and  the  plant 
will  be  fertilized  by  the  pollen  of  its  own 
species.  He  had  shown  that  the  two  forms  of 
flowers  of  the  cowslip  were  beautifully  adapted 
in  structure  for  cross-fertilization,  and  that  it 
was  essential  to  the  vigor  of  the  species  that 
there  be  cross-fertilization  between  the  two 
forms,  and  that  cross-fertilization  between 
flowers  of  the  same  form,  or  fertilization  of  a 

1  Different  Forms  of  Flowers,  etc.,  p.  28. 


144  THE  METHOD   OF  DARWIN. 

flower  by  its  own  pollen,  should  be  prevented. 
But  he  had  also  shown  that  there  is  a  mechan- 
ical liability  to  self-fertilization  in  the  short- 
styled  form  by  the  pollen  being  carried  down 
from  the  stamens  to  the  pistil.  These  facts 
led  him  to  the  belief  that  the  pollen  of  the 
other  form  is  prepotent  over  the  pollen  of  the 
same  form  as  that  to  which  the  stigma  belongs, 
when  they  are  placed  on  it  together.  He  said, 
"  There  can  hardly  be  a  doubt  that  with  hetero- 
styled  dimorphic  plants,  pollen  from  the  other 
form  will  obliterate  the  effects  of  the  pollen 
from  the  same  form,  even  when  this  has  been 
placed  on  the  stigma  a  considerable  time  before. 
To  test  this  belief  I  placed  on  several  stigmas 
of  a  long-styled  cowslip  plenty  of  pollen  from 
the  same  plant,  and  twenty  hours  later  pollen 
from  a  short -styled  dark  red  polyanthus"  (va- 
riety of  cowslip).  Of  the  thirty  seedlings,  all 
bore  reddish  flowers,  showing  that  they  were 
the  result  of  the  cross.1 

It  is  not  surprising  that,  after  he  had  verified 
his  general  inference  concerning  prepotency  in 
heterostyled  dimorphic  plants  by  the  experi- 
ments with  the  highly  specialized  flowers  of 
the  cowslip,  he  should  take  another  step  and 
test  the  inference  by  a  species  in  which  there 

1  Different  Forms  of  Flowers,  etc,  p.  31. 


DED  UC  TION.  —  A  NTICIPA  TION.  1 4  5 

was  much  less  difference  between  the  two 
forms.  After  describing  the  two  forms  of 
Linum  grandiflorum,  which  differ  only  in  the 
length  of  their  pistils,  he  said  that  he  had  in 
his  garden  two  fine  long-styled  plants,  sep- 
arated by  a  considerable  distance  and  some 
evergreens  from  plants  of  the  other  form.  He 
marked  twelve  flowers  on  these  two  long-styled 
plants,  and  put  some  short-styled  pollen  on 
them ;  but  they,  and  the  rest  of  the  vast  num- 
ber of  flowers  on  these  two  plants,  had  their 
stigmas  already  covered  with  their  own  pollen, 
and  it  was  late  in  the  season  (September  15). 
The  pollen  of  the  two  forms  could  not  be  distin- 
guished under  the  microscope.  "Altogether," 
said  Darwin,  "it  seemed  almost  childish  to 
expect  any  result.  Nevertheless,  from  my  ex- 
periments on  Primula  I  had  faith,  and  did  not 
hesitate  to  make  the  trial,  but  certainly  did 
not  anticipate  the  full  result  that  was  obtained. 
The  germens  of  the  twelve  flowers  swelled,  and 
bore  six  good  capsules  (the  seed  of  which  germi- 
nated) and  two  bad  ones;  only  four  falling  off. 
All  the  other  flowers  were  absolutely  barren; 
not  even  their  germens  swelled."1  Not  con- 
tent with  this  striking  result,  he  hunted  down 
this  absolute  sterility  of  the  long-styled  plants 

1  Different  Forms  of  Flowers,  etc.,  p.  83. 


146  THE  METHOD  OF  DARWIN. 

with  their  own  form  pollen,  which  his  experi- 
ments of  1 86 1  had  brought  out,  by  carrying 
through  a  long  series  of  experiments  which,  as 
he  said,  were  so  curious  that  he  gave  them  in 
detail.  His  conclusion  from  the  experiments  is 
that  although  the  pollen  of  the  two  forms  is  iden- 
tical under  the  microscope,  "taking  fertility  as 
a  criterion  of  distinctness,  it  is  no  exaggeration 
to  say  that  the  pollen  has  been  brought  to  a  de- 
gree of  differentiation,  with  respect  to  its  action 
on  the  stigma  of  the  same  form,  corresponding 
with  that  existing  between  the  pollen  and  stigma 
of  species  belonging  to  distinct  genera."  l 

An  excellent  illustration  of  the  relation 
between  induction  and  deduction  in  the  scien- 
tific method  is  furnished  by  Darwin's  study  of 
the  expression  of  grief.  During  several  years, 
he  said,  no  expression  seemed  to  him  so  utterly 
perplexing  as  this  one.  Why  should  the  inner 
ends  of  the  eyebrows  be  raised  when  a  person 
feels  the  emotion  of  grief?  Or,  in  other  words, 
what  is  the  cause  of  the  obliquity  of  the  eye- 
brows under  suffering?  "Why  should  grief  or 
anxiety  cause  the  central  fasciae  alone  of  the 
frontal  muscle,  together  with  those  round  the 
eyes,  to  contract?"2  Here  was  a  little  induc- 

1  Different  Forms  of  Flowers,  etc.,  pp.  87-90. 

2  Expression  of  the  Emotions,  etc.,  pp.  188-192. 


DED  UCTION.  —  ANTIC  IP  A  TION.  1 47 

tive  problem:  the  effects  given,  to  find  the 
cause,  without  any  hint  as  to  its  nature.  Sev- 
eral years  would  seem  long  enough  for  a  mind 
like  Darwin's. 

He  had  seen  Duchenne's  photograph  of  a 
young  man  contracting  the  grief-muscles  while 
looking  up  at  a  strongly  illuminated  surface, 
but  had  entirely  forgotten  the  picture.  The 
problem  remained  unsolved.  One  day  while 
on  horseback,  with  the  sun  behind  him,  he  met 
a  young  girl  whose  eyebrows  as  she  looked 
up  at  him  became  extremely  oblique,  with  the 
proper  furrows  on  her  forehead.  He  had  been 
unable  to  reach  the  cause  by  contemplation 
of  the  effects.  Now  he  had  observed  a  cause 
producing  effects  identical  with  those  which 
puzzled  him.  There  was  now  a  basis  for  ex- 
periment He  went  home  and  caused  three  of 
his  children,  without  their  knowing  why,  to 
look  up  at  the  top  of  a  tall  pine  against  a 
bright  sky.  "With  all  three,"  he  said,  "the 
orbicular,  corrugator,  and  pyramidal  muscles 
were  energetically  contracted,  through  reflex 
action  "  to  protect  the  eyes  against  the  light. 
But  they  wanted  to  see,  and  a  "  curious  struggle 
arose  between  these  muscles,  that  tended  to 
lower  the  brows  and  close  the  lids  and  all  or 
only  the  central  fasciae  of  the  frontal  muscle, 


148  THE  METHOD   OF  DARWIN. 

causing  obliquity  of  the  brows  with  puckering 
and  swelling  of  their  inner  ends,  so  that  the 
exact  expression,  in  every  detail,  of  grief  or 
anxiety  was  assumed  " 

"When  children  scream  they  contract  the 
orbicular,  corrugator,  and  pyramidal  muscles, 
primarily  for  the  sake  of  compressing  their 
eyes,  and  thus  protecting  them  from  being 
gorged  with  blood."  By  observation  on  the 
little  girl  and  his  own  children  he  found  that 
the  peculiar  expression  characteristic  of  grief 
was  the  combined  result  of  the  reflex  contrac- 
tion of  the  muscles  around  the  eye  to  protect  it 
by  closing  it,  and  the  voluntary  contraction  of 
other  muscles  to  keep  it  open.  In  children  the 
unrestrained  expression  of  suffering  called  into 
action  only  one  set  of  these  muscles.  These 
were  the  factors  in  his  possession  for  the  solu- 
tion of  the  problem.  He  knew  that  the  eleva- 
tion of  the  inner  ends  of  the  eyebrows  was  due 
to  the  effort  to  keep  the  eyes  open.  "  I  there- 
fore expected  to  find  with  children,"  he  said, 
"that  when  they  endeavored  either  to  prevent 
a  crying-fit  from  coming  on,  or  to  stop  crying, 
they  would  check  the  contraction  of  the  above- 
named  muscles,  in  the  same  manner  as  when 
looking  upwards  at  a  bright  light;  and  conse- 
quently that  the  central  fasciae  of  the  frontal 


DEDUCTION.  —  ANTICIPA  TION.  149 

muscle  would  often  be  brought  into  play."  He 
himself  observed  children  with  reference  to 
this  point,  and  had  others,  including  physi- 
cians, do  so.  He  soon  found  that  the  "grief- 
muscles  "  were  very  frequently  brought  into 
very  distinct  action,  and  has  given  a  number 
of  cases.  The  crying-muscles  act  in  children, 
and  have  acted  for  countless  generations.  The 
pyramidal  muscles  are  least  under  the  control 
of  the  will,  and  can  be  counteracted  only  by 
the  voluntary  contraction  of  the  central  fasciae 
of  the  frontal  muscle ;  and  that  is  the  expression 
of  grief. 

This  instance  of  the  solution  of  an  inductive 
problem  is  by  its  apparent  smallness  a  striking 
example  of  the  difficulties  of  scientific  investi- 
gation, and  of  the  necessity  of  appealing  to  all 
the  logical  processes.  After  the  explanation  is 
once  made,  it  would  seem  an  easy  matter  to  ana- 
lyze the  complex  effect  called  the  expression 
of  grief,  and  discover  its  causes  in  the  involun- 
tary contraction  of  one  set,  and  the  voluntary 
contraction  of  another  set  of  muscles;  but  the 
apparent  simplicity  is  all  due  to  the  explana- 
tion itself.  It  will  be  remarked,  in  the  discus- 
sion of  the  logical  history  of  the  principle  of 
natural  selection,  that  the  causes  of  any  set  of 
facts  are  rarely  discovered  by  a  direct  study 


I5O  THE  METHOD  OF  DARWIN. 

of  the  effects.  Sooner  or  later  a  cause  is  ob- 
served to  produce  the  effect  of  which  an  ex- 
planation is  desired;  then  by  an  induction  the 
cause  is  applied  to  the  whole  class  of  effects, 
and  the  induction  is  established  by  subsequent 
deduction  and  verification. 

One  of  the  most  interesting  examples  of 
reasoning  recorded  in  Darwin's  works  is  that 
concerning  the  colors  of  caterpillars.  Although 
Darwin  put  the  problem  and  Wallace  solved  it, 
it  is  inserted  here  because  it  illustrates  the 
devices  used  to  secure  explanations  of  facts. 
Darwin  had  undertaken  to  explain  the  beauty 
of  butterflies  by  the  principle  of  sexual  selec- 
tion, but  knew  that  it  was  foolish  to  think  that 
the  beauty  of  the  mature  animal  was  thus  ex- 
plained unless  the  equally  beautiful  colors  of 
many  caterpillars,  in  whose  case  sexual  selec- 
tion certainly  could  not  act,  were  explained  in 
some  special  way.1  Here  was  another  set  of 
facts  that  could  not  be  reduced  under  his 
theory;  and  again  appears  the  almost  insuper- 
able difficulty  of  passing  directly  from  facts  to 
their  causes.  He  whom  many  regarded  as  a 
master  in  the  art  of  "wriggling"  was  unable  to 
devise  an  explanation,  and  appealed  to  Wallace, 

1  Descent  of  Man,  etc.,  Vol.  I.  pp.  202-204 ;  Life  and  Letters, 
Vol.  II.  p.  276. 


DED  UC  TION.  —  A  N  TIC  IP  A  TION.  1 5  I 

who,  he  said,  had  an  innate  genius  for  solving 
difficulties.  Wallace,  by  a  pretty  paradox,  re- 
ferred this  anomalous  set  of  facts  to  the  well 
known  principle  of  protective  coloring.  He 
said  most  caterpillars  require  protection,  as 
may  be  inferred  from  some  kinds  having  spines 
or  hairs,  others  being  colored  like  leaves  and 
twigs.  But  in  known  cases  in  which  the  color- 
ing served  to  protect  the  larva  it  did  so  by  con- 
cealing the  animal  from  its  enemies.  Gaudy 
colors  would  expose  it  to  the  sharp  sight  of 
every  foe ;  how,  then,  could  they  serve  to  pro- 
tect it?  By  reasoning  from  these  considera- 
tions "Mr.  Wallace  thought  it  probable  that 
conspicuously  colored  caterpillars  were  pro- 
tected by  having  a  nauseous  taste."  But  as 
they  are  tender,  a  peck  from  a  bird's  beak 
would  be  fatal.  "Distastefulness  alone,"  said 
Wallace,  "would  be  insufficient  to  protect  a 
caterpillar  unless  some  outward  sign  indicated 
to  its  would-be  destroyer  that  its  -prey  was 
a  disgusting  morsel."  Darwin  presented  the 
reasoning  to  the  Entomological  Society,  and 
it  was  supported  by  statements  of  the  members, 
and  afterwards  verified  by  Jenner  Weir's  ex- 
periments with  his  birds.  The  smooth  green 
and  twig-like  larvae  were  devoured  by  the 
birds;  and  the  spinous  and  brightly  colored 


152  THE   METHOD  OF  DARWIN. 

ones  were  rejected  with  signs  of  distaste- 
fulness. 

The  correlation  of  brilliant  colors  and  dis- 
tastefulness  was  thus  anticipated  as  a  conse- 
quence of  natural  selection,  the  principle  of 
protective  coloring  and  the  belief  that  animals 
so  protected  by  obscure  colors  are  attractive  to 
their  enemies.  Darwin  immediately  seized  on 
this  result  as  an  opening  for  further  investiga- 
tions, and  said :  "  This  view  will,  it  is  probable, 
be  hereafter  extended  to  many  animals  which 
are  colored  in  a  conspicuous  manner."  The 
study  of  animal  coloration  under  Darwin's 
principle  of  natural  selection  and  the  subordi- 
nate principles  of  mimicry,  etc.,  has  been  car- 
ried to  such  a  length,  or  rather  the  effort  to 
explain  coloration  under  these  and  similar  prin- 
ciples has  been  carried  to  such  a  length,  that 
one  prominent  zoologist  has  felt  justified  in 
characterizing  the  speculations  of  recent  years 
on  the  coloration  of  animals  as  a  mild  form  of 
scientific  lunacy.  There  has  been  an  enormous 
amount  of  wild  deduction  and  half-digested 
observation ;  but  what  is  most  needed  is  more 
light  on  the  physiological  causes  at  work  within 
the  animal  and  producing  and  determining  the 
distribution  of  colors. 

It  has  been  mentioned  in  another  place  that 


DED  UCTION.  —  ANTICIPA  TION.  1 5  3 

Darwin  had  concluded  that  characters  inherited 
by  both  sexes  appear  early  in  life,  and  that  char- 
acters restricted  to  one  sex  appear  late.  He 
said,  "  I  was  led  to  infer  that  a  relation  of  this 
kind  exists,  from  the  fact  that  whenever  and  in 
whatever  manner  the  adult  male  has  come  to 
differ  from  the  adult  female,  he  differs  in  the 
same  manner  from  the  young  of  both  sexes." 
This  induction  he  proceeded  to  strengthen  by 
deduction  at  both  ends.  He  deduced  the  law 
from  a  still  more  general  one,  as  follows :  "  It 
is  in  itself  probable  that  any  character  appear- 
ing at  an  early  age  would  tend  to  be  inherited 
equally  by  both  sexes,  for  the  sexes  do  not 
differ  much  in  constitution,  before  the  power 
of  reproduction  is  gained  " ;  and  in  the  same 
way  characters  appearing  late  in  one  sex  would 
tend  to  be  restricted  to  that  sex. l 

From  the  other  side,  he  sought  to  test  the 
law  by  an  examination  of  its  consequences;  and 
chose  the  deer  family  as  a  crucial  instance,  upon 
which  he  felt  he  could  rely.2  In  some  species 
of  this  family  the  males  alone  have  horns,  and 
in  one  species  both  sexes  have  them.  If  the 
law  were  true,  horns  should  appear  late  in 
the  species  in  which  they  are  restricted  to  the 

1  Descent  of  Man,  etc.,  Vol.  I.  p.  276. 

2  Ibid.,  p.  278  et  seq. 


154  THE  METHOD  OF  DARWIN. 

males,  and  early  in  the  species  of  which  both 
sexes  have  horns.  He  found  that  in  seven 
species  with  horns  only  on  the  males  the  horns 
appear  late,  —  at  nine  months,  or  even  later. 
In  the  reindeer,  which  alone  has  horns  on  both 
sexes,  the  horns  appear  in  both  sexes  at  the  age 
of  four  or  five  weeks.  His  investigations  took 
a  somewhat  wider  range,  and  he  gave  other 
illustrations  and  some  exceptions  to  the  law 
among  horned  animals. 

One  of  the  most  striking  of  his  minor  efforts 
was  his  explanation  of  the  origin  of  the  remark- 
able color  patterns  called  "  ocelli  "  on  the  tail 
of  the  peacock.1  He  had  adopted  the  theory 
of  sexual  selection  for  the  explanation  of  the 
beauty  of  birds,  etc.,  which  did  not  seem  to 
him  to  admit  of  explanation  under  natural  selec- 
tion. There  are  other  naturalists  who  do  not 
believe  that  sexual  selection  plays  the  part  in 
nature  which  Darwin  ascribed  to  it.  But  what- 
ever view  is  taken  of  it,  it  furnished  him  with 
the  indispensable  working  hypothesis  by  means 
of  which  to  explain  known  facts  and  search  for 
others  that  ought  to  follow  as  consequences  of 
the  hypothesis.  It  was  characteristic  of  Dar- 
win to  select  for  investigation  and  explanation 
extreme  instances  that  would  put  his  beliefs  to 

1  Descent  of  Man,  etc.,  Vol  II.  pp.  132-145. 


DED  UCTION.  —  ANTIC  IP  A  TION.  I  5  5 

the  most  rigorous  test.  The  ocelli  on  the  tail 
of  the  peacock  furnished  such  an  instance.  The 
problem  was  how  to  explain  the  origin  of  these 
ocelli  from  the  ordinary  feather  markings  of 
the  group  to  which  the  peacock  belongs. 

First,  his  belief  that  the  colotf  patterns  of 
many  birds  are  due  to  sexual  selection  led  him 
to  explain  the  absence  of  ocelli  from  the  tail- 
feathers  of  the  two  species  of  peacock  by  the 
fact  that  these  feathers  are  covered  up  and  con- 
cealed by  the  long  tail-coverts.  The  theory 
required  that  the  patterns  due  to  sexual  selec- 
tion should  be  exposed  to  the  gaze  of  the 
females.  "In  this  respect,"  he  said,  "they 
differ  remarkably  from  the  tail-feathers  of  Poly- 
plectron,  which  in  most  of  the  species  are  orna- 
mented with  larger  ocelli  than  those  on  the 
tail-coverts."  According  to  theory,  the  ocelli 
had  disappeared  from  the  tail-feathers  and 
been  remarkably  developed  on  the  tail-coverts 
because  the  latter  by  enormous  development 
had  covered  the  former.  In  Polyplectron,  in 
which  the  tail-coverts  are  not  so  enormously 
developed,  the  ocelli  were  larger  on  the  tail- 
feathers  than  on  the  coverts.  By  the  theory, 
however,  the  markings  of  the  Polyplectron  are 
more  generalized,  and  some  indications  of 
gradations  between  these  two  extremes  ought 


156  THE  METHOD   OF  DARWIN. 

to  exist.  "  Hence,"  he  said,  "  I  was  led  to  care- 
fully examine  the  tail-feathers  of  the  several 
species  of  Polyplectron  in  order  to  discover 
whether  the  ocelli  in  any  of  them  showed  any 
tendency  to  disappear,  and,  to  my  great  satis- 
faction, I  was  successful."  His  whole  study  of 
these  ocelli  is  remarkable  for  the  ingenuity 
with  which  he  worked  out  the  consequences  of 
his  theory  and  verified  them ;  and  explained  the 
intricate  pattern  as  a  modification  and  speciali- 
zation of  more  general  feather-marking. 

Darwin's  theory  of  descent  led  him  to  regard 
species  as  only  more  strongly  marked  varieties. 
"From  looking  at  species  as  only  strongly 
marked  and  well  defined  varieties,  I  was  led  to 
anticipate  that  the  species  of  the  larger  genera 
in  each  country  would  oftener  present  varieties 
than  the  species  of  the  smaller  genera."  He 
tested  this  deduction  by  an  extensive  tabulation 
of  plants  and  Coleoptera;  "and  it  has  invariably 
proved  to  be  the  case  that  a  larger  proportion 
of  the  species  on  the  side  of  the  larger  genera 
presented  varieties  than  on  the  side  of  the 
smaller  genera. "  1 

When  once  he  had  got  hold  of  natural  selec- 
tion as  the  cause  that  had  brought  about  the 
adaptations  in  nature,  it  was  an  essential  part  of 

1  Origin  of  Species,  p.  44. 


DED  UCTION.  —  A  NTICJPA  TION.  1 5  / 

the  doctrine  that  only  variations  favorable  to 
the  species  could  be  preserved.  It  was  an 
inevitable  inference  from  the  nature  of  the 
cause  of  adaptations  that  there  could  be  no 
such  thing  in  nature  as  an  adaptation  for  the 
exclusive  benefit  of  any  other  species  than  the 
one  possessing  it.  Darwin  was  so  confident 
of  this  deduction,  although  it  could  never  be 
rigidly  verified  by  observation,  that  he  boldly 
staked  the  fate  of  his  whole  theory  on  the 
truth  of  the  inference.  "  If  it  could  be  proved 
that  any  part  of  the  structure  of  any  one  species 
had  been  formed  for  the  exclusive  good  of 
another  species,  it  would  annihilate  my  theory, 
for  such  could  not  have  been  produced  through 
natural  selection."1  He  could  never  know 
from  direct  observation  that  there  were  no  such 
cases  in  existence,  but  he  was  driven  to  the 
assertion  of  their  non-existence  by  the  nature 
of  the  cause  of  adaptations.  Doubtless  this 
deduction  gave  the  severest  blow  that  was  ever 
dealt  to  the  belief  in  general  benevolence  in 
Nature,  and  it  has  proved  the  final  blow.  It 
was  the  apparently  awful  consequences  of  this 
inevitable  deduction  that  roused  the  bitter 
opposition  of  the  religious  forces  of  the  world. 
It  seemed  to  establish  forever  the  doctrine  that 

1  Origin  of  Species,  p.  162. 


158  THE  METHOD   OF  DARWIN. 

advantage  to  self  is  the  only  invariable  motive 
of  all  the  striving  in  the  universe.  The  moral 
consequences  of  the  theory  seemed  to  outrage 
all  the  noble  ideals  that  had  ever  been  cherished 
in  the  world. 

Since  the  time  he  made  the  declaration,  there 
have  been  up  for  discussion  numerous  cases  of 
adaptation  which  seemed  of  no  value,  or  even 
hurtful,  to  the  species  possessing  them.  These 
difficulties  have  caused  a  vast  amount  of  stren- 
uous explanatory  wriggling  under  the  name 
symbiosis;  but  there  has  as  yet  been  no  case 
found  which  can  be  positively  regarded  as  an 
example  of  what  Darwin  said  could  not  exist. 
He  had  long  studied  adaptations  carefully;  but 
the  nature  of  the  cause,  after  he  had  discovered 
it,  helped  him  to  understand  more  clearly  the 
nature  of  adaptations. 

Good  testimony  for  the  necessity  of  a  theory 
of  some  kind  to  enable  him  to  work  effectively 
is  what  Darwin  wrote  to  Asa  Gray  about 
cleistogamic  flowers,  which  are  self-fertilized 
and  do  not  open  at  all.  He  said,  "The  tem- 
porary theory  which  I  have  formed  on  this  class 
of  dimorphism,  just  to  guide  experiment,  is 
that  the  perfect  flowers  can  only  be  perfectly 
fertilized  by  insects,  and  are  in  this  case  abun- 
dantly crossed;  but  that  the  flowers  are  not 


DED  UCTION.  —  ANTICIPA  TION.  1 5  9 

always,  especially  in  early  spring,  visited 
enough  by  insects,  and  therefore  the  little 
imperfect  self-fertilizing  flowers  are  developed 
to  insure  a  sufficiency  of  seed  for  present  genera- 
tions. "  This  temporary  theory  is  now  generally 
accepted  as  the  true  one.1 

1  Life  and  Letters,  Vol.  II.  p.  482. 


XL 

DEDUCTION.  — GENERAL   INSTANCES. 

IT  has  been  said  that  a  large  proportion  of 
Darwin's  special  investigations  were  started 
as  deductions  from  the  general  theories  of 
descent  and  natural  selection  for  the  purpose 
of  corroborating  or  testing  them,  and  otherwise 
working  out  their  consequences.  But  before 
turning  to  these  it  will  be  well  to  notice  a 
piece  of  deductive  work  not  connected  with 
those  theories;  for  it  is  important,  both  on 
account  of  his  using  it  himself  as  an  illustra- 
tion of  his  method  and  on  account  of  the  more 
recent  views  that  have  arisen  in  opposition 
to  it. 

The  hypothesis  concerning  the  formation  of 
coral  reefs  took  its  rise  as  a  special  application 
of  the  general  rule  that  depositions  are  made 
over  sinking  areas  to  the  coral  limestones  of 
the  west  coast  of  South  America. 

The  main  outlines  of  his  work  on  coral 
islands  were  deductive,  and  so  were  many  of 
the  special  investigations  of  details.  He  spoke 


DEDUCTION.  — GENERAL   INSTANCES.     l6l 

of  the  theory  of  the  formation  of  coral  islands 
as  the  only  one  of  his  many  hypotheses  which 
he  had  not  to  modify  afterwards,  and  asserted 
that  therefore  his  study  of  coral  islands  was 
more  deductive  in  spirit  than  any  other  of  his 
investigations.1  After  the  theory  had  held 
sway  for  forty  years  in  the  geological  world  as 
a  complete  explanation  of  coral  reefs,  it  was 
called  in  question  by  John  Murray,  who  sought 
to  replace  it  by  another  which  seemed  to  him 
more  reasonable.  Murray  has  found  some  ad- 
herents; the  war  of  the  two  theories  has  been 
waged  more  or  less  hotly  ever  since,  and  both 
views  are  still  in  the  field.  Darwin's  theory 
is  not  superseded,2  nor  is  it  likely  to  be;  but 
it  will  nevertheless  be  somewhat  modified  to 
adjust  it  to  the  greater  knowledge  'of  the 
present.  Indeed,  if  Darwin  had  lived  to  issue 
another  edition  of  his  book,  he  would  have 
taken  the  new  facts  into  account,  as  he  did  in 
his  latest  correspondence.  Here,  again,  he  had 
considered  in  advance  many  of  the  objections 
that  have  since  been  raised  against  his  theory. 
But  the  chief  interest  of  this  case  is  derived 

1  Life  and  Letters,  Vol.  I.  pp.  58,  83. 

2  Structure   and   Distribution  of   Coral  Reefs,  3d  edition, 
Appendix  by  Prof.  T.  G.  Bonney;  Ibid.,   Bettany's  edition, 
Critical  Introduction  by  Prof.  John  W.  Judd. 

ii 


1 62  THE  METHOD   OF  DARWIN. 

from  the  facts  that  he  regarded  it  as  the  most 
deductive  in  spirit  of  all  his  work,  and  as  the 
only  hypothesis  which  he  was  not  obliged  to 
modify,  and  that  scientists  have  since  attacked 
it  more  severely  perhaps  than  any  of  his  other 
theories. 

Darwin's  approach  to  the  subject  of  the  ex- 
pression of  the  emotions  has  already  been 
described  under  Induction.  He  said,  "When  I 
read  Sir  C.  Bell's  great  work  [On  the  Anat- 
omy of  Expression],  his  view,  that  man  had 
been  created  with  certain  muscles  specially 
adapted  for  the  expression  of  his  feelings, 
struck  me  as  unsatisfactory."1  This  was  be- 
cause he  had  become  convinced  of  the  truth  of 
evolution,  which  required  him  to  believe  that 
the  habit  of  expressing  our  feelings  by  cer- 
tain movements  had  been  somehow  gradually 
acquired.  This  view  required  that  the  whole 
subject  of  expression  should  be  studied  under 
a  new  aspect;  "and  each  expression,"  he  said, 
"demanded  a  rational  explanation.  This  belief 
led  me  to  attempt  the  present  work."  In  this 
case  the  general  theory  led  him  only  to  the 
conviction  that  there  must  be  some  rational 
explanation  for  each  emotional  expression;  and 
left  him  to  find  out  inductively  the  particular 

1  Expression  of  the  Emotions,  p.  19. 


DEDUCTION.— GENERAL   INSTANCES.     163 

nature  of  the  explanations.  It  will  be  recalled 
that  he  reached  three  principles  of  expres- 
sion; but  only  when  he  had  completed  his 
observations. 

Darwin's  botanical  work  was  almost  entirely 
done  under  the  influence  of  the  theory  of  evolu- 
tion. His  special  investigations  in  this  field 
were  based  on  corollaries  from  the  general 
theory.  Writing  to  Mr.  Murray,  his  publisher, 
concerning  his  book  on  the  "Fertilization  of 
Orchids,"  he  said,  "It  will  perhaps  serve  to 
illustrate  how  natural  history  may  be  worked 
under  the  belief  of  the  modification  of  species. "  1 
Equipped  with  the  belief  that  all  adaptations 
are  useful  to  the  species  possessing  them, 
and  that  innumerable  flowers,  among  them  the 
orchids,  are  adapted  for  cross-fertilization,  he 
studied  orchids  and  the  effects  of  cross-  and 
self-fertilization  in  the  same  spirit  in  which  he 
had  studied  coral  islands. 

One  of  the  most  interesting  of  the  many 
investigations  that  arose  out  of  his  general 
theories  was  that  on  climbing  plants.  As  has 
been  said,  he  selected  as  subjects  for  special 
research  some  of  the  principal  difficulties  that 
presented  themselves  for  explanation.  He  did 
this  because,  if  the  theories  were  true,  they 

1  Life  and  Letters,  Vol.  II.,  Letter  to  Murray. 


1 64  THE   METHOD   OF  DARWIN. 

ought  to  lead  to  the  right  explanations,  and 
because  he  always  regarded  it  as  better  to 
work  out  a  single  typical  case  or  a  few  typ- 
ical cases  thoroughly,  in  proof  of  his  theories, 
than  to  offer  miscellaneous  suggestions  on 
many  cases. 

Climbing  plants  offered  a  case  of  special 
importance.  When  once  the  principles  of 
descent  and  natural  selection  are  adopted  as 
working  hypotheses,  it  is  a  comparatively  easy 
matter  to  explain  how  a  group  of  closely  related 
animals  or  plants  come  to  possess  one  or  more 
striking  characteristics  in  common.  In  such  a 
case  they  are  all  supposed  to  have  inherited  the 
characters  in  question  from  a  common  ancestor. 
For  example,  all  the  woodpeckers,  with  their 
peculiar  feet  and  tail  and  tongue,  the  last 
with  its  remarkable  apparatus  of  hyoid-bone 
and  muscle,  —  or  the  various  oaks  with  their 
acorns  and  cupules,  their  flowers  and  leaves, 
—  are  believed  to  have  descended  from  a  com- 
mon stock.  The  individuals  of  these  groups 
have  a  great  many  characters  in  common.  But 
climbing  plants  are  found  throughout  the  plant 
kingdom.  Some  families  contain  several  or 
many  closely  related  climbers,  and  these  offer 
no  special  difficulty  so  far  as  their  mutual  pos- 
session of  the  power  to  climb  is  concerned. 


DEDUCTION.— GENERAL   INSTANCES.     165 

But  other  large  families,  normally  composed  of 
non-climbers,  contain  one  or  a  few  climbing 
species,  as,  for  example,  the  hop  in  the  nettle 
family. 

The  power  to  climb  is  so  striking  a  charac- 
ter, and  is  so  plainly  useful  to  the  plant  pos- 
sessing it,  that  Darwin's  theories  would  be 
taxed  with  failure  if  they  did  not  explain  its 
origin.  But  climbing  plants  are  found  through- 
out the  plant  kingdom ;  and  they  are  not 
descended  from  a  common  climbing  ancestor, 
for  they  possess  nothing  in  common  except 
this  one  power  to  climb.  ''Plants, "he  said, 
"become  climbers  in  order,  it  may  be  pre- 
sumed, to  reach  the  light,  and  to  expose  a 
large  surface  of  leaves  to  its  action  and  to 
that  of  the  free  air.  This  is  effected  by  the 
climbers  with  wonderfully  little  expenditure  of 
organized  matter  in  comparison  with  trees, 
which  have  to  support  a  load  of  heavy  branches 
by  a  massive  trunk.  Hence,  no  doubt,  it  arises 
that  there  are  in  all  quarters  of  the  world  so 
many  climbing  plants  belonging  to  so  many 
different  orders."  The  very  great  advantage 
offered  to  the  climber  has  acted  as  a  powerful 
premium  for  the  development  of  the  capacity 

1  Journal  of  the  Linnean  Society,  1865;  Botany,  Vol.  IX. 
pp.  107,  108. 


1 66  THE  METHOD   OF  DARWIN. 

wherever  variation  offered  the  materials  out  of 
which  natural  selection  could  produce  it. 

The  first  thing  to  be  established  in  proof  of 
the  derivation  of  climbing  plants  from  non- 
climbers  was  the  existence  of  gradations  in  the 
power  of  climbing,  and  of  intermediate  stages 
between  the  different  methods  of  climbing,  — 
by  twining  of  the  stem,  by  leaf-stalks,  and  by 
tendrils;  just  as  he  connected  the  ocelli  of  the 
peacock's  tail  by  a  series  of  gradations  with 
the  more  ordinary  feather-markings  of  related 
birds.  But  another  unknown  element  was  the 
source  of  the  variations  upon  which  natural 
selection  could  work  to  produce  climbers.  In 
his  arguments  to  prove  his  theories  of  descent 
and  natural  selection  Darwin  showed  that 
variations  do  occur,  and  that  when  they  occur 
natural  selection  will  inevitably  preserve  the 
favorable  and  destroy  the  unfavorable.  But  he 
could  do  little  or  nothing  in  the  direction  of 
pointing  out  the  cause  of  variations.  He  has 
been  incessantly  twitted  about  this  by  his 
opponents,  especially  because  of  the  false 
notion  that  he  ascribed  to  chance  all  variations 
whose  causes  were  not  known.  Ignorance  of 
the  sources  of  variation  is  no  obstacle  what- 
ever to  belief  in  Darwin's  theories;  but  this 
is  true  only  when  the  species  having  a  certain 


DEDUCTION.  — GENERAL  INSTANCES.     1 6? 

character  in  common  are  proved  by  many  other 
characters  to  be  descended  from  a  common 
stock.  In  the  case  of  climbing  plants  the  same 
kind  of  variation  must  have  occurred  indepen- 
dently in  all  parts  of  the  plant  kingdom,  or 
there  must  have  been  a  common  source  or  ten- 
dency which  served  as  a  starting  point  for  the 
development  of  the  power  to  climb. 

In  his  work  on  climbing  plants,  first  pub- 
lished as  a  paper  in  the  Linnean  Journal,1  he 
worked  out  in  a  masterly  way  the  gradations  in 
the  power  to  climb,  and  between  the  different 
methods  of  climbing,  showing  that  all  are  mod- 
ifications of  the  method  of  climbing  by  twining 
of  the  stem/  Near  the  end  of  his  work  he  said, 
"We  have  seen  how  diversified  are  the  move- 
ments of  climbing  plants.  .  .  .  They  belong  to 
many  and  widely  different  orders.  .  .  .  When 
we  reflect  on  this  wide  serial  distribution  of 
plants  having  this  power,  and  when  we  know 
that  in  some  of  the  largest  well  defined  orders, 
such  as  the  Compos itae,  Rubiaceae,  Scrophu- 
lariaceae,  Liliaceae,  etc.,  two  or  three  genera 
alone  out  of  a  host  of  genera  in  each,  have  this 
power,  the  conclusion  is  forced  on  our  minds 
that  the  capacity  of  acquiring  the  revolving 

1  Journal  of  the  Linnean  Society,  1865,  Botany,  Vol.  IX., 
"  On  the  Movements  and  Habits  of  Climbing  Plants." 


1 68  THE   METHOD   OF  DARWIN. 

power  on  which  most  climbers  depend  is 
inherent  though  undeveloped  in  almost  every 
plant  in  the  vegetable  kingdom."1 

It  will  be  interesting  to  try  to  analyze  the 
conditions  under  which  he  made  this  definite 
prediction.  In  his  work  on  climbing  plants  he 
showed  that  the  power  to  climb  depends  on  two 
quite  distinct  powers:  (i)  the  power  of  spon- 
taneous circumnutation,  and  (2)  sensitiveness  to 
touch,  and  the  consequent  bending  toward  the 
side  touched. 

Without  the  theory  of  descent,  the  question 
of  the  origin  of  the  above  mentioned  sensitive- 
ness could  never  have  arisen  at  all.  With  the 
theory  of  descent,  and  with  natural  selection  as 
a  cause,  and  a  belief  in  the  existence  of  varia- 
tions for  it  to  work  upon,  it  might  have  been 
possible  to  infer  some  general  power  or  tendency 
in  plants  as  the  source  of  that  sensitiveness  to 
touch;  but  it  was  not  done,  and  from  what  has 
been  said  elsewhere  it  is  not  likely  that  it 
would  have  been  done,  at  least  without  great 
difficulty,  from  a  knowledge  of  the  highly 
specialized  effects.  Darwin  said,  "  If  we  in- 
quire how  the  petiole  of  a  leaf,  or  the  peduncle 
of  a  flower,  or  a  branch,  first  becomes  sensitive, 
and  acquires  the  power  of  bending  toward  the 

1  Journal,  p.  117. 


DEDUCTION.  — GENERAL   INSTANCES.     169 

touched  side,  we  get  no  certain  answer.  Nev- 
ertheless, an  observation  by  Hofmeister  well 
deserves  attention,  namely,  that  the  shoots  and 
leaves  of  all  plants,  whilst  young,  move  after 
being  shaken;  and  it  is  almost  invariably  young 
petioles  and  young  tendrils,  whether  formed  of 
modified  leaves  or  flower-peduncles,  which  move 
on  being  touched;  so  that  it  would  appear  as 
if  these  plants  had  utilized  and  perfected  a 
widely  distributed  and  incipient  capacity,  which 
capacity,  as  far  as  we  can  see,  is  of  no  service  to 
ordinary  plants."  l 

Darwin  was  in  search  of  a  source  of  the  sen- 
sitiveness of  plants,  and  Hofmeister  had  pro- 
vided it  by  empirical  observation.  Darwin's 
relation  to  this  explanation  was  exactly  the 
same  as  it  was  in  the  discovery  of  the  principle 
of  natural  selection.  It  will  be  seen  that  in 
the  latter  case  he  had  studied  very  carefully  the 
effects  (adaptations)  to  be  accounted  for,  and 
variations  as  the  material  upon  which  the  un- 
known cause  might  act;  then  by  accidental 
reading  of  Malthus  the  cause  was  presented 
to  him,  and  he  brought  it  and  its  effects  into 
relation  with  each  other  by  interpreting  the 
latter  as  results  of  the  action  of  the  former.  In 
the  case  of  the  sensitiveness  to  touch  in  plants, 

1  Journal,  p.  112. 


THE  METHOD   OF  DARWTN. 

he  was  in  possession  of  the  cause  (natural  selec- 
tion), and  the  specialized  effects  and  the  belief 
that  these  are  the  results  of  the  action  of  natural 
selection  upon  variations ;  Hofmeister  furnished 
the  material  which  natural  selection  could  de- 
velop, by  the  observation  that  all  young  plants 
are  slightly  sensitive  to  disturbance. 

Now  what  was  the  logical  setting  for  Darwin's 
prediction  that  the  capacity  of  revolving  would 
be  found  inherent,  though  undeveloped  in  almost 
every  plant  in  the  vegetable  kingdom  ?  If  there 
were  no  evidence  to  the  contrary,  it  would  be 
supposed  that  he  had  been  able  to  make  the 
prediction  from  the  knowledge  that  climbing 
plants  occur  throughout  the  plant  kingdom, 
and  that  therefore  the  source  of  the  variation 
must  be  a  general  tendency  in  plants.  Such  an 
inference  seems  easy  enough  to  make,  as  has 
been  shown  in  other  cases,  after  it  has  been 
made.  In  this  case  Darwin  had  the  help  of  an 
analogy  upon  which  he  could  depend  with  con- 
fidence. Hofmeister  had  furnished,  by  obser- 
vation, a  general  source  of  the  sensitiveness  of 
climbing  plants  in  the  slight  sensitiveness  of 
young  leaves  and  shoots  in  general.  Sensitive- 
ness to  touch  and  power  of  circumnutation  are 
inseparable  in  climbing  plants;  what  was  more 
natural,  therefore,  than  the  inference  that  the 


DEDUCTION.— GENERAL  INSTANCES.     17 1 

power  to  revolve  had  its  source  in  an  unknown 
general  tendency,  just  as  sensitiveness  to  touch 
had  its  source  in  a  known  slight  general  sen- 
sitiveness, each  power  having  been  developed 
and  specialized  in  climbing  plants  by  natural 
selection?  Such  an  inference  would  seem  al- 
most inevitable.  But  in  addition  he  was  in 
actual  possession  of  a  case  in  which  the  power 
of  revolving  was  imperfect  and  functionless. 

He  said  that  when  he  made  the  prediction  he 
"knew  of  only  one  imperfect  case,  namely,  of 
the  young  flower-peduncles  of  a  Maurandia^ 
which  revolved  slightly  and  irregularly,  like 
the  stems  of  twining  plants,  but  without  mak- 
ing any  use  of  the  habit."  l  In  the  discussion 
of  Maurandia  semperflorens,  in  his  original 
paper  on  climbing  plants,  he  gave  the  follow- 
ing interesting  bit  of  history :  "  I  should  not 
have  noticed  the  present  species,  had  it  not 
been  for  the  following  unique  case.  Mohl  says 
that  the  flower-peduncles,  as  well  as  the  petioles, 
are  wound  into  tendrils."  Darwin  proved  that 
the  flower-peduncles  do  not  act  as  tendrils ;  but 
that  they  nevertheless,  whilst  young,  exhibit 
feeble  revolving  powers,  and  are  slightly  sen- 
sitive to  a  touch.  He  observed  nine  vigorous 
plants,  and  it  was  certain  to  him  that  neither 

1  Origin  of  Species,  p.  197. 


THE  METHOD   OF  DARWIN. 

the  slight  spontaneous  movements  nor  the  slight 
sensitiveness  of  the  flower-peduncles  were  of 
any  service  to  the  plants  in  climbing.  He 
gave  reasons  for  believing  that  these  imperfect 
powers  are  not  relics  of  former  functional  power, 
and  that  correlation  of  growth  did  not  transfer 
them  imperfectly  from  the  internodes  and  young 
petioles  to  the  flower-peduncles;  and  said  that, 
by  whatever  means  acquired,  the  case  was  inter- 
esting to  him  because  these  useless  capaci- 
ties, by  being  a  little  perfected,  would  make 
the  flower-peduncles  of  this  plant  as  useful  for 
climbing  as  are  those  of  Vitis  and  other  plants.1 
The  important  part  which  the  case  of  Mau- 
randia  played  in  the  reasoning  is  shown  by 
what  he  said  concerning  it.  What  he  had  said 
of  the  source  of  sensitiveness  to  touch  he  almost 
literally  repeated  of  the  power  of  circumnuta- 
tion :  "  If  we  further  inquire  how  the  stems  of 
petioles,  tendrils,  and  flower-peduncles  of  climb- 
ing plants  first  acquire  their  power  of  spontane- 
ously revolving,  or,  to  speak  more  accurately, 
of  successively  bending  to  all  points  of  the 
compass,  we  are  again  silenced,  or  at  most  can 
only  remark  that  the  power  of  movement,  both 
spontaneous  and  from  various  stimuli,  is  far 

1  Journal   of  the  Linnean  Society,  1865;  Botany,  Vol.  IX. 
pp.  38-40. 


DEDUCTION.  — GENERAL   INSTANCES.     1/3 

more  common  with  plants,  as  we  shall  presently 
see,  than  is  generally  supposed  to  be  the  case 
by  those  who  have  not  attended  to  the  subject. 
There  is,  however,  the  one  remarkable  case  of 
the  Maurandia  semperflorens,  in  which  the 
young  flower-peduncles  spontaneously  revolve 
in  very  small  circles,  and  bend  themselves  when 
gently  rubbed  to  the  touched  side;  yet  this 
plant  profits  in  no  way  by  these  two  feebly 
developed  powers.  A  rigorous  examination  of 
other  young  plants  would  probably  show  some 
slight  spontaneous  movements  in  the  peduncles 
and  petioles,  as  well  as  that  sensitiveness  to 
shaking  observed  by  Hofmeister.  We  see,  at 
least  in  the  Maurandia,  a  plant  which  might, 
by  a  little  augmentation  of  qualities  which  it 
already  possesses,  come  first  to  grasp  a  support 
by  the  flower-peduncles,  as  with  Vitis  and  Car- 
diospermum,  and  then  by  the  abortion  of  some 
of  its  flowers  acquire  perfect  tendrils."1  At 
this  point  he  made  the  prediction  already 
quoted. 

To  sum  up,  Darwin  based  his  conclusions 
concerning  the  source  of  the  power  of  revolving 
upon  the  following  data:  (i)  detailed  knowledge 
of  the  nature  and  extent  of  the  climbing  power 
in  the  plant  kingdom ;  (2)  proof  that  there  are 

1  Journal,  p.  113. 


174  THE  METHOD   OF  DARWIN. 

many  gradations  of  structure  and  function 
between  simple  twiners  and  tendril  bearers; 
(3)  the  conviction  that  all  such  highly  special- 
ized functions  and  structures  are  extreme  de- 
velopments of  more  general  but  less  obvious 
phenomena,  or  of  slight  variations  whose  source 
is  unknown ;  (4)  the  almost  perfectly  analogous 
case  of  sensitiveness  to  touch  in  climbing  plants, 
which  is  inseparably  connected  with  the  power 
of  revolving,  and  which  he  had  connected  with 
the  general  though  slight  sensitiveness  of  plants 
observed  by  Hofmeister;  (5)  and  one  of  the 
most  important  of  the  data  mentioned,  the 
actual  case  of  imperfect  and  functionless  power 
of  revolving  in  Maurandia  semperflorens,  which 
might  by  a  little  augmentation  become  useful 
in  climbing. 

The  final  verification  of  the  prediction  •  is 
embodied  in  the  volume  on  the  "  Power  of 
Movement  in  Plants,"  by  Charles  and  Francis 
Darwin.  The  sweeping  character  of  the  verifi- 
cation cannot  be  better  indicated  than  by  quot- 
ing their  own  statement  of  what  they  intended 
to  show  in  that  volume ;  it  will  serve,  too,  as  a 
summary  of  results,  for  they  completely  estab- 
lished what  they  claimed :  "  In  the  course  of 
the  present  volume  it  will  be  shown  that  ap- 
parently every  growing  part  of  every  plant  is 


DEDUCTION,— GENERAL  INSTANCES.     175 

continually  circumnutating,  though  often  on  a 
small  scale.  Even  the  stems  of  seedlings  be- 
fore they  have  broken  through  the  ground,  as 
well  as  the  buried  radicles,  circumnutate  as  far 
as  the  pressure  of  the  surrounding  earth  permits. 
In  this  universally  present  movement  we  have 
the  basis  or  groundwork  for  the  acquirement, 
according  to  the  requirements  of  the  plant,  of 
the  most  diversified  movements.  Thus  the  great 
sweeps  made  by  the  stems  of  twining  plants  and 
by  the  tendrils  of  other  climbers  result  from  a 
mere  increase  in  the  amplitude  of  the  ordinary 
movements  of  circumnutation.  The  position 
which  young  leaves  and  other  organs  ultimately 
assume  is  acquired  by  the  circumnutating  move- 
ment being  increased  in  some  one  direction. 
The  leaves  of  various  plants  are  said  to  sleep 
at  night,  and  it  will  be'  seen  that  their  blades 
then  assume  a  vertical  position  through  modified 
circumnutation,  in  order  to  protect  their  upper 
surfaces  from  being  chilled  through  radiation. 
The  movements  of  various  organs  to  the  light, 
which  are  so  general  throughout  the  vegetable 
kingdom,  and  occasionally  from  the  light,  or 
transversely  with  respect  to  it,  are  all  modified 
forms  of  circumnutation;  as,  again,  are  the 
equally  prevalent  movements  of  stems,  etc. 
towards  the  zenith,  and  of  roots  towards  the 


1 76  THE  METHOD   OF  DARWIN. 

centre  of  the  earth.  In  accordance  with  these 
conclusions,  a  considerable  difficulty  in  the  way 
of  evolution  is  in  part  removed,  for  it  might 
have  been  asked,  How  did  all  their  diversified 
movements  for  the  most  different  purposes  first 
arise  ?  As  the  case  stands,  we  know  that  there 
is  always  movement  in  progress,  and  its  ampli- 
tude or  direction,  or  both,  have  only  to  be  mod- 
ified for  the  good  of  the  plant  in  relation  with 
internal  or  external  stimuli. "  1 

Thus  the  great  work  of  observation  and  rea- 
soning began  with  an  effort  to  explain  the  power 
of  climbing  among  plants  under  the  theories  of 
descent  and  natural  selection;  passed  on  to  the 
prediction  of  the  universal  movement  of  cir- 
cumnutation  and  its  verification;  and  closed  by 
explaining  all  the  other  highly  specialized  and 
remarkable  movements 'of  plants  and  plant  or- 
gans as  modifications  of  the  same  general  but 
unapparent  movement.  The  principal  difficulty 
at  first  was  the  fact  that  climbers  were  found 
throughout  the  plant  kingdom,  and  could  not 
have  been  descended  from  a  common  climbing 
ancestor.  By  the  investigations  of  Darwin  and 
his  son,  not  only  were  the  different  methods  of 
climbing  shown  to  be  modifications  of  the  twin- 
ing movements  of  the  stem,  but  it  and  all  the 

1  Power  of  Movement  in  Plants,  p.  4. 


DEDUCTION.— GENERAL   INSTANCES. 

other  movements  of  plants  were  shown  to  be 
modifications  of  a  universal  movement.  What 
was  at  first  a  difficulty  in  the  way  of  evolu- 
tion became,  like  the  structure  of  the  flowers 
of  orchids,  the  ocelli  of  the  peacock,,  and  the  ex- 
pression of  the  emotions,  one  of  the  strongest 
supports  of  the  theory. 


12 


XII. 

UNVERIFIED   DEDUCTIONS. 

EVERY  apparently  insignificant  fact  was 
full  of  meaning  to  Darwin;  and  he  made 
it  the  occasion  for  what  he  used  to  call  "fool's 
experiments."  His  speculative  powers  em- 
ployed themselves  as  actively  and  energetically 
on  the  details  of  his  investigations  as  on  their 
larger  outlines;  but  he  was  as  ruthless  in  test- 
ing and  rejecting  his  speculations  as  he  was 
facile  in  making  them.  When,  however,  he 
had  once  established  a  principle,  he  followed 
out  the  deductions  from  it  with  as  much  confi- 
dence as  if  he  had  already  secretly  seen  the 
facts  whose  existence  he  suspected  or  thought 
probable.  It  is  important  to  note  the  caution 
with  which  he  usually  stated  his  anticipations, 
and  to  contrast  with  it  the  energy  and  confi- 
dence with  which  he  sought  and  worked  out 
the  facts.  He  spent  his  life  establishing  the 
consequences  of  his  theories,  but  with  all  his 
fidelity  and  persistence  he  had  to  leave  many 
things  unproved;  some  for  lack  of  time,  others 


UNVERIFIED  DEDUCTIONS.  1 79 

because  of  the  inaccessibility  of  the  facts.  He 
had  the  satisfaction  of  living  to  see  the  whole 
biological  world  applying  itself  to  the  work  of 
bringing  out  the  consequences  of  his  theories. 
It  is  not  a  part  of  the  present  purpose,  even  if  it 
were  possible,  to  follow  out  the  logical  history 
of  the  subsequent  work  based  on  those  theo- 
ries. But  it  will  be  of  interest  to  notice  a  few 
instances  in  which  he  made  deductions  which 
he  could  not  verify.  Some  of  these  have  been 
since  verified  by  others,  and  some  still  remain 
unverified.  They  vary  all  the  way  from  confi- 
dent predictions  to  vague  expressions  of  a  wish 
that  some  one  would  make  observations  that  he 
thought  would  bear  fruit. 

There  is  one  instance  of  a  difficulty  in  the 
way  of  Darwin's  theories  which  is  of  especial 
importance.  He  outlined  a  possible  explana- 
tion, but  the  difficulty  has  proved  itself  so  stub- 
born that  even  some  of  his  adherents  feel  that 
his  theories  could  not  face  many  of  the  same 
kind.  The  problem  is  almost  exactly  similar 
to  that  of  climbing  plants,  and  its  interest  is 
increased  by  some  recent  work  that  has  been 
done  towards  its  solution. 

Electric  organs  occur  in  various  unrelated 
species  of  fishes,  and  differ  so  widely  in  their 
position  in  the  body,  their  mode  of  innervation, 


ISO  THE  METHOD   OF  DARWIN. 

etc.,  that  they  could  not  possibly  have  been 
derived  from  a  single  primitive  electric  organ. 
In  this  respect  the  difficulty  was  exactly  similar 
to  that  of  the  climbing  power  in  plants.  After 
pointing  out  that  the  electric  organs  of  the 
different  electric  fishes  are  not  homologous,  — 
that  they  occupy  different  parts  of  the  body, 
are  differently  innervated,  etc.,  —  Darwin  said 
that  the  problem  which  remains  is  "by  what 
graduated  steps  these  organs  have  been  devel- 
oped in  each  separate  group  of  fishes. "  In  the 
case  of  climbing  plants  he  had  been  able  to 
show  many  gradations  of  structure  and  function, 
—  that  even  when  not  fully  developed  the  power 
to  climb  was  serviceable  to  plants.  He  said, 
"  The  electric  organs  of  fishes  offer  another  case 
of  special  difficulty;  for  it  is  impossible  to  con- 
ceive by  what  steps  these  wondrous  organs  have 
been  produced.  But  this  is  not  surprising,  for 
we  do  not  even  know  of  what  use  they  are."  l 

Darwin  mentioned,  however,  as  factors  for 
the  solution  of  the  problem,  the  great  differ- 
ences in  the  strength  of  the  shocks,  the  close 
analogy,  as  he  called  it,  between  the  electric 
organs  and  muscular  tissue,  the  electrical  phe- 
nomena of  ordinary  muscle;  and  called  atten- 
tion to  our  ignorance  of  the  habits  and  structure 

1  Origin  of  Species,  pp.  150,  151. 


UNVERIFIED  DEDUCTIONS.  l8l 

of  the  progenitors  of  electric  fishes,  and  added, 
"  It  would  be  extremely  bold  to  maintain  that 
no  serviceable  transitions  are  possible  by  which 
these  organs  might  have  been  gradually  devel- 
oped." Nevertheless,  the  subject  was  unap- 
proachable for  him;  and  his  opponents  have 
used  the  electric  fishes  as  one  of  the  greatest 
stumbling-blocks  in  the  way  of  natural  selec- 
tion. Even  Romanes  felt  that  the  electric 
fishes  present  so  serious  an  obstacle  that  if 
there  were  many  such  he  would  have  to  hold  in 
abeyance  his  belief  in  the  theory.1  Darwin 
believed  that  the  facts,  because  they  could  not 
be  explained,  did  not  therefore  militate  against 
the  theory.  The  much  greater  difficulty  of 
explaining  the  case  of  the  electric  organs  com- 
pared with  that  of  climbing  plants  is  due  to  two 
important  facts.  In  the  latter  there  were  at 
least  some  gradations  of  structure  and  function 
known ;  and  even  when  both  were  imperfect,  it 
could  be  shown  that  they  were  useful,  so  that 
natural  selection  could  act  upon  them.  But 
there  were  no  imperfectly  developed  electric 
organs  in  fishes  which  could  be  appealed  to  as 
the  source  from  which  the  perfect  organs  might 
be  developed;  and  what  was  still  more  impor- 
tant, in  some  of  the  electric  fishes  the  organs, 

1  Romanes,  Darwin  and  After  Darwin. 


1 82  THE  METHOD   OF  DARWIN. 

though  perfect  in  structure,  were  apparently 
functionless;  they  were  not  known  to  give  off 
any  electrical  discharge. 

Darwin  left  the  problem  unsolved  except  for 
the  suggestions  he  offered,  and  it  still  remains 
unsolved  in  part.  Among  the  reasons  for  this 
state  of  the  subject  are  these :  that  the  habits  of 
the  electric  fishes  have  been  but  little  known, 
that  the  powers  of  the  electric  organs  are  but  lit- 
tle understood,  and  that  both  are  very  difficult 
to  investigate. 

The  present  state  of  the  subject  is  made 
interesting  by  the  recent  studies  of  Professor 
Fritsch.1  From  the  first  the  electric  organs 
have  been  regarded  as  modified  muscular  tissue. 
They  occupy  the  place  of  what  in  other  fishes 
is  common  muscle.  The  Pacinian  law  of  the 
direction  of  the  current  in  electric  organs  -is 
that  at  the  instant  of  the  shock  the  side  of  the 
electric  plate  on  which  the  nerve  enters  is  neg- 
ative, and  the  opposite  side  positive;  and  in 
this  important  respect  such  of  them  as  are  of 
undoubted  muscular  origin  agree  with  the  com- 
mon muscular  tissue  and  its  electrical  phenom- 
ena. Fritsch' s  interesting  recent  studies  on 

1  Gustav  Fritsch,  Archiv  fiir  Anatomic  und  Physiologic, 
Supplement  Band,  1892;  Nature,  Jan.  19,  1893,  Vo1-  XLVII. 
p.  271. 


UNVERIFIED  DEDUCTIONS.  183 

the  Mormyrida  of  the  Nile  have  removed  the 
objection  based  on  the  existence  of  structurally 
perfect  electric  organs  which  do  not  give  off 
electric  discharges.  He  showed  that  these  deli- 
cate little  fishes,  hitherto  regarded  as  pseudo- 
electric,  are  really  capable  of  giving  off  shocks 
that  can  be  felt  by  the  hand  and  can  be  easily 
measured  with  instruments.  He  has  also  shown 
that  in  these  fishes  a  part  of  the  organ  is  in 
a  state  of  transition  from  muscular  to  electric 
tissue.  At  the  posterior  end  the  electric  organ 
is  sharply  set  off  from  the  muscular  tissue; 
but  at  the  anterior  end  it  graduates  more  or 
less  perfectly  into  the  muscular  tissue,  and  the 
organ  seems  to  be  actually  developing  in  that 
direction. 

A  special  difficulty  lay  in  the  fact  that  in 
MalapteruruS)  an  African  fish,  in  which  the 
electric  organ  lies  beneath  the  skin  and  encases 
the  body  like  a  sheath,  the  electric  current  does 
not  obey  the  Pacinian  law,  but  takes  the  oppo- 
site direction.  Fritsch  removed  the  whole  diffi- 
culty by  giving  reasons  for  the  belief  that  in 
this  case  the  organ  is  not  of  muscular  but  of 
cutaneous  origin,  and  represents  the  cutaneous 
glands  that  are  so  plentiful  in  some  parts  of 
the  body.  Such  an  exception,  without  the  ex- 
planation of  Fritsch,  would  confuse  the  whole 


1 84  THE  METHOD   OF  DARWIN. 

problem  of  the  relation  between  muscle  and 
the  electric  organs.  With  the  explanation,  it 
furnishes  a  good  example  of  how  investigation 
dissolves  difficulties. 

It  is  a  common  saying  that  the  solution  of 
one  problem  leads  to  new  and  frequently  more 
difficult  ones.  It  may  almost  be  said  an  inves- 
tigation should  be  viewed  with  suspicion  if  it 
does  not  leave  more  new  problems  than  it  started 
out  to  solve.  The  belief  of  Fritsch  that  the 
electric  organs  of  Malapterurus  are  homologous 
with  the  cutaneous  glands  opened  up  a  new 
phase  of  the  general  problem  of  the  origin  of 
electric  organs.  Another  difficulty  presented 
itself  to  Fritsch  in  the  process  of  solving  the 
original  problem  which  he  set  before  himself. 
In  his  study  of  the  innervation  of  the  electric 
organs  of  Mormyrus  he  found  that  the  nerves 
which  supply  the  electric  organs  decussate,  or 
cross  from  one  side  of  the  body  to  the  other, 
after  leaving  the  spinal  cord  as  anterior  roots, 
—  much  as  the  optic  nerves  form  the  optic 
chiasma  within  the  brain-case.  There  is  no 
similar  case  known  of  nerve  fibres  crossing 
from  one  side  of  the  body  to  the  other  after 
they  have  left  the  central  nervous  system ;  and 
Fritsch  properly  thinks  it  to  be  more  difficult 
to  explain  by  gradual  variation  and  natural 


UNVERIFIED  DEDUCTIONS.  185 

selection  than  the  origin  of  the  electric  organs 
themselves.  He  has  suggested  that  this  re- 
markable peculiarity  has  been  developed  to 
insure  perfect  co-ordination  between  the  elec- 
tric organs  of  the  two  sides  of  the  body,  with- 
out which  there  could  not  be  perfect  unity  of 
action  in  the  electric  discharge,  whereas  while 
the  tissue  is  merely  muscular  it  is  important 
that  the  organs  of  the  two  sides  should  act 
independently  of  each  other. 

Fritsch  has  also  called  attention  to  the  fact 
that  the  sluggish  powerfully  electric  fishes  are 
carnivorous,  and  the  active  feebly  electric  spe- 
cies are  at  least  mostly  herbivorous. 

More  recently  still,  Professor  Engelmann,1 
of  Utrecht,  in  a  histological  study  of  the  elec- 
tric organs  in  embryos  of  Raja  clavata,  etc., 
has  established  in  detail  the  genetic  relations 
between  some  of  the  elements  of  cross-striped 
muscle-fibre  and  the  lamellae  of  the  so  called 
meandrine  or  striped  layer  which,  lying  be- 
neath the  nerve  end-plate  in  most  species  of 
Raja,  forms  one  of  the  principal  constituents  of 
the  electric  organs  in  the  tail  of  these  fishes. 
Hitherto  little  more  had  been  known  than  that 

1  Th.  W.  Engelmann,  Archiv  fur  die  Gesammte  Physiolo- 
gic des  Menschen  und  der  Thiere,  Band  LVII.  pp.  149-180, 
June  1 6,  1894. 


1 86  THE  METHOD   OF  DARWIN. 

such  relations  existed.  The  whole  problem  has 
advanced  much  farther  toward  a  solution  on  the 
morphological  than  on  the  physiological  side; 
but,  as  might  have  been  expected,  the  demon- 
stration of  the  exact  relations  between  the 
various  elements  of  muscle-fibre  and  those  of 
the  electric  organs  has  enabled  Engelmann  to 
indicate  some  very  important  and  fruitful  lines 
for  physiological  experiment. 

Although  the  homology  of  cross-striped  mus- 
cle-fibre and  the  constituents  of  the  electric 
organ  has  been  proved  even  to  many  histo- 
logical  details,  and  the  morphological  changes 
by  which  the  former  has  been  transformed  into 
the  latter  have  become  quite  clear,  the  diffi- 
culty of  explaining  the  development  of  electric 
organs  from  muscular  tissue  by  means  of  natural 
selection  is  as  great  as  ever.  Engelmann  has 
significantly  suggested  the  existence  of  func- 
tions that  we  are  entirely  ignorant  of;  and 
refers,  by  way  of  illustration,  to  the  recent 
remarkable  discoveries  of  the  effects  of  the 
removal,  partial  and  complete,  and  grafting  of 
the  thyroid  gland,1  and  to  Brown-Sequard's 
demonstration  of  the  existence  of  an  extremely 
important  "internal  secretion"  in  the  renal  and 

1  Archives  de  Physiologic  Normale  et  Pathologique,  1890- 
1894,  and  elsewhere. 


UNVERIFIED  DEDUCTIONS.  187 

other  glands.1  He  has  marked  out  a  new  field 
of  physiological  research  on  the  problem  of 
electric  organs  by  the  suggestion  that  the 
marine  biological  laboratories  be  taken  advan- 
tage of  to  study  the  effects  which  the  partial  or 
total  removal,  or  destruction,  transplantation, 
etc.  of  those  organs  in  various  stages  of  onto- 
genetic  and  phylogenetic  development  will  have 
on  the  normal  functions  of  the  animal.  He 
believes  that  it  might  thus  be  quickly  shown 
that  these  so  called  useless  organs  do  serve  an 
important  function  in  the  animal  economy,  and 
that  it  might  be  made  clear  what  that  function 
is ;  or  that  it  might  at  least  be  shown  what  injury 
their  absence  entails  upon  the  animal. 

It  must  be  remembered  that  the  problem  of 
electric  organs  has  not  yet  been  attacked  in  any 
such  way  as  that  in  which  Darwin  attacked  the 
problem  of  climbing  plants.  Fritsch  by  two 
brief  visits  brought  to  light  many  important 
facts;  on  the  morphological  side  contributions 
are  steadily  made,  but  the  knowledge  on  the 
subject  up  to  date  has  been  brought  out  piece- 
meal. Before  the  mode  of  development  of  elec- 
tric organs  can  be  fully  understood,  there  will 
have  to  be  not  only  knowledge  of  the  electrical 

1  Ibid.,  Series  V.,  Vol.  V.  pp.  778-786,  October,  1893,  and 
elsewhere. 


1 88  THE  METHOD  OF  DARWIW. 

phenomena  of  muscle  in  general,  but  a  thorough 
investigation  of  the  normal  electrical  phenom- 
ena of  fish-muscle,  and  of  the  efficacy  of  slight 
shocks  in  water;  the  habits  and  environments 
of  the  electric  fishes,  their  enemies  and  their 
prey,  need  to  be  carefully  studied.  When  the 
investigation  is  set  going  on  a  large  enough 
scale,  and  along  all  the  lines  on  which  there 
is  at  present  little  more  than  dense  ignorance, 
there  will  be  in  the  minds  of  those  who  have 
attended  closely  to  the  principal  biological 
investigations  of  recent  years  no  doubt  about 
the  outcome.  The  logical  processes  involved 
in  the  solution  of  such  problems  under  the 
influence  of  a  general  theory  are  practically  the 
same,  whether  the  work  is  done  by  one  man  or 
by  a  number  of  men  who  attack  the  problems 
simultaneously  or  in  succession. 

The  unverified  deduction  in  the  following 
example  is  interesting  on  several  accounts, 
(i)  The  prediction  involved  what  seemed  to 
many  entomologists  an  improbability.  (2)  The 
prediction  has  not  been  verified,  but  the  im- 
probability has  been  removed.  (3)  It  has  been 
often  quoted  even  by  naturalists  as  a  case  of 
verified  prediction.  Angrczcum  scsquipedale, 
an  orchid  native  to  Madagascar,  has  a  long 
whip-like  nectary.  On  several  flowers  -Darwin 


UNVERIFIED  DEDUCTIONS.  189 

found  it  eleven  and  a  half  inches  long,  with 
only  an  inch  and  a  half  of  nectar  at  the  bottom. 
He  proved  to  his  own  satisfaction,  by  a  study  of 
the  structure  of  the  flower,  that  it  is  fertilized 
by  moths  that  thrust  their  proboscides  and  heads 
down  into  the  flower  to  the  utmost,  and  declared 
that  moths  with  proboscides  long  enough  to 
reach  the  nectar  must  exist  in  Madagascar. 
"This  belief  of  mine,"  he  said,  "has  been  ridi- 
culed by  some  entomologists ;  but  we  now  know 
from  Fritz  Miiller  that  there  is  a  sphinx  moth 
in  South  Brazil  which  has  a  proboscis  of  nearly 
sufficient  length,  for  when  dried  it  was  between 
ten  and  eleven  inches  long.  When  not  pro- 
truded, it  is  coiled  up  into  a  spiral  of  at  least 
twenty  windings."  The  moth  with  the  long 
proboscis  has  not  yet  been  discovered  in  Mada- 
gascar; but  the  fact  that  there  are  moths  else- 
where with  proboscides  as  long  as  the  one 
required  in- the  case  of  Angrcecum  has  removed 
the  improbability  from  the  prediction.  But  the 
point  to  be  borne  in  mind  is  this,  that  the  firm 
conviction  that  orchids  are  fertilized  by  insects, 
and  that  the  flowers  and  insects  are  co -adapted 
in  structure,  led  Darwin  to  believe  firmly  in  an 
"  improbable  thing.*' l 

The  questions  raised   in  Darwin's   mind  by 

1  Fertilization  of  Orchids,  pp.  162-166. 


I QO  THE  METHOD   OF  DARWIN. 

his  own  theories  were  innumerable;  some  of 
them  he  answered  by  the  great  investigations 
already  mentioned,  others  he  left  unanswered 
for  various  reasons.  Only  about  two  months 
before  his  death  he  pointed  out  new  lines  of 
investigation.  Among  other  things  he  said 
that  there  were  many  inconspicuous  flowers  not 
known  to  be  visited  by  insects  during  the  day, 
and  the  natural  inference  is  that  they  are  self- 
fertilized.  And  he  pointed  out  the  desirability 
of  finding  out  whether  these  flowers  are  visited 
at  night  by  the  innumerable  individuals  of  the 
many  species  of  minute  moths.  If  they  are  not 
so  visited,  why  do  they  expand  at  all  ?  Why 
are  they  not  cleistogamic  ?  He  suggested,  as  a 
mode  of  procedure,  smearing  the  flowers  with 
viscid  matter  and  then  looking  for  insect  scales; 
but  gave  the  caution  that  it  would  be  necessary 
to  prove  that  the  matter  employed  was  not  in 
itself  attractive  to  insects.1 

It  is  a  fascinating  study  to  follow  out  the 
suggestions  that  came  to  him  and  that  he  made 
to  others,  to  note  the  various  degrees  of  success 
with  which  the  investigations  were  made  by 
others,  to  compare  the  spirit  and  methods  with 
which  they  were  made  with  Darwin's  own  spirit 

1  Miiller,  Fertilization  of  Flowers,  Prefatory  Note,  by 
Charles  Darwin. 


UNVERIFIED  DEDUCTIONS.  IQI 

and  method.  He  knew  the  importance  of  study- 
ing the  speech  of  monkeys  in  relation  to  his 
belief  in  the  descent  of  man,  and  expected  valu- 
able results  from  it.  "I  wish,"  he  wrote  to 
Asa  Gray,  "  some  one  would  keep  a  lot  of  the 
noisiest  monkeys,  half  free,  and  study  their 
means  of  communication."1  Mr.  Garner  has 
recently  written  magazine  articles  and  a  book 
on  the  subject,  and  has  even  visited  the  ape 
country  in  Africa  with  elaborate  arrangements 
for  studying  the  speech  of  monkeys  in  their 
native  haunts.  It  is  too  early  to  forecast  re- 
sults, but  this  case  illustrates  well  how  different 
phases  of  Darwin's  theories  have  attracted  dif- 
ferent types  of  men,  and  how  caution  or  the 
want  of  it  may  make  or  break  confidence  in  the 
results  of  their  investigations. 

1  Life  and  Letters,  Vol.  II.  p.  183. 


XIII. 

ERRONEOUS   DEDUCTION. 

CONSIDERED  simply  as  a  logical  process, 
^-^  deduction  is  no  more  interesting  in  the 
hands  of  the  modern  investigator  than  it  was 
in  the  hands  of  the  mediaeval  schoolman.  The 
scientist  uses  it,  apart  from  its  importance  in 
proof,  or  effort  to  convince  others,  merely  as  an 
instrument  with  which  to  test  what  is  known, 
and  to  develop  its  unknown  consequences.  The 
infallibility  of  the  process  is  altogether  hypo- 
thetical. The  conclusion  is  true  only  if  "the 
premises  are  true;  and  since  the  truth  of  the 
premises  is  oftener  the  matter  in  question  than 
even  the  investigator  dreams,  the  effort  to  get 
at  new  truth  by  anticipating  the  consequences 
of  theory  often  results  in  false  conclusions. 
This  must  especially  be  the  case  when  there  is 
no  apparent  reason  to  question  the  truth  of  the 
premises ;  when  they  would  seem  to  have  been 
permanently  established  by  repeated  crucial 
tests.  In  a  number  of  instances  Darwin  went 


TNIVERSI 
ERRONEOUS  DEDUCTION-. 

clearly  wrong  in  his  deductions.  Some  of  them 
he  corrected  himself,  for  some  he  accepted  the 
corrections  of  others ;  and  some  have  never  yet 
been  corrected  directly,  but  only  by  the  adop- 
tion of  the  contrary  principle  from  a  study  of 
facts  similar  to  those  on  which  Darwin  went 
wrong.  When  his  attention  had  become  fixed 
upon  the  cowslip  (Primula  veris),  he  found  that 
there  were  two  forms  of  flowers  on  plants  of 
this  species.  He  said,  "The  first  idea  which 
naturally  occurred  to  me  was,  that  this  species 
was  tending  towards  a  dioecious  condition;  that 
the  long-styled  plants,  with  their  longer  pistils, 
rougher  stigmas,  and  smaller  pollen  grains,  were 
more  feminine  in  nature,  and  would  produce 
more  seed;  that  the  short-styled  plants,  with 
their  shorter  pistils,  longer  stamens,  and  larger 
pollen  grains,  were  more  masculine  in  nature."1 
Nothing  would  seem  more  natural  than  that  the 
structural  differences  between  the  flowers  should 
indicate  differences  in  sexual  function.  The 
knowledge  of  dioecious  plants  and  belief  in  the 
modification  of  species  could  plainly  suggest  but 
one  interpretation  of  the  structural  differences  of 
the  two  kinds  of  flowers.  Either  they  indicated 
what  was  inferred  from  them,  or  they  could 

1  Different   Forms   of    Flowers   on    Plants    of    the    same 
Species,  pp.  18-21. 

13 


194  THE  METHOD   OF  DARWIN. 

indicate  nothing  at  all.  Had  his  conclusion 
been  left  at  this  point,  it  would  probably  have 
been  accepted  as  both  interesting  and  quite  cer- 
tain. The  degree  of  certitude  with  which  such 
an  inference  is  received  depends  on  the  number 
of  facts  involved,  their  relation  to  each  other, 
and  the  degree  to  which  they  act  as  convergent 
evidence  toward  the  one  conclusion.  In  these 
respects  the  facts  were  all  that  could  be  desired. 
It  would  seem  that,  if  it  is  possible  to  make  any 
inferences  at  all  concerning  function  from  the 
structure  of  plants,  it  would  have  been  so  in  this 
case. 

For  Darwin,  as  for  every  true  student  of 
nature,  deductions  exist  only  to  be  verified. 
The  indirect  evidence  from  structure  he  supple- 
mented by  experiments  on  the  actual  produc- 
tion of  seed  by  the  two  forms.  He  might  have 
pointed  with  pride  to  the  cowslip  as  a  plant  in 
an  actual  state  of  transition, —  as  a  fine  illus- 
tration of  his  theory.  But  after  describing  in 
detail  the  differences  of  structure  in  the  two 
forms  of  flower  in  the  cowslip,  he  said,  "The 
question  seems  well  worthy  of  careful  investi- 
gation." He  made  preliminary  experiments 
which  of  themselves  would  have  been  conclu- 
sive, but  used  them  to  lay  a  basis  for  his  much 
more  extensive  experiments;  they  suggested 


ERRONEOUS  DEDUCTION.  195 

methods,  furnished  cautions,  and  brought  out 
facts  enough  to  determine  the  most  important 
directions  of  investigation.  He  marked  a  few 
plants  of  each  form  in  his  garden,  in  a  field, 
and  in  a  shady  wood,  and  gathered  and  weighed 
the  seed.  In  all  the  lots  the  short-styled  plants 
yielded,  contrary  to  his  expectation,  most  seed. 
He  gave  tables  of  results,  and  added  that  "by 
all  these  standards  of  comparison  the  short- 
styled  form  is  the  more  fertile."  In  1861  he 
made  fuller  and  fairer  trials,  and  found  that 
the  same  result  also  held  good  for  some  other 
species  of  Primula.  "  Consequently  my  anticipa- 
tion that  the  plants  with  longer  pistils,  rougher 
stigmas,  shorter  stamens,  and  smaller  pollen 
grains  would  prove  to  be  more  feminine  in 
nature,  is  exactly  the  reverse  of  the  truth." 

The  facts  on  which  his  first  inference  was 
based  were  easy  to  observe;  they  corroborated 
each  other  in  a  remarkable  way,  and  harmonized 
perfectly  with  what  was  well  known  concerning 
dioecious  plants.  There  was  no  reason  to  sus- 
pend judgment,  for  there  were  no  facts  that 
obtruded  themselves  as  objections  to  the  infer- 
ence. Now  if  under  such  circumstances  an 
inference  turns  out  to  be  exactly  the  reverse  of 
the  truth,  what  guaranty  is  there  that  a  conclu- 
sion will  be  correct  in  any  case  ?  The  answer 


196  THE  METHOD   OF  DARWIN. 

was  given  in  Darwin's  researches.  The  evi- 
dence was  not  all  in.  The  structural  evidence, 
instead  of  serving  as  a  basis  for  a  true  inference 
concerning  the  functions  of  the  organs,  was 
found  to  be  in  contradiction  with  them.  Darwin 
showed  that  both  sets  of  facts  were  dependent 
on  a  hitherto  altogether  unknown  phenomenon, 
the  differentiation  of  the  flowers  of  a  species 
into  distinct  sets  with  marked  structural  differ- 
ences for  the  purpose  of  making  cross-fertiliza- 
tion almost  absolutely  certain. 

The  following  case  is  perhaps  as  interesting 
as  it  could  be  made,  to  illustrate  the  danger  of 
accepting  as  truth  inferences  that  fall  short  of 
demonstration,  and  only  represent  a  high  degree 
of  probability.  In  all  the  books  that  Darwin 
had  consulted  Enonymus  Europeans  (spindle- 
tree)  is  called  hermaphrodite.  But  he  found 
from  an  examination  of  the  species  that  about 
half  of  the  individuals  had  both  stamens  and 
pistils  of  normal  size,  and  were  therefore  her- 
maphrodite; and  that  the  remaining  half  had 
pistils  of  the  normal  size,  but  short  stamens  with 
rudimentary  anthers  without  pollen,  and  were 
therefore  properly  females.  The  ovules  were 
of  equal  size  in  the  two  forms.  There  could 
not  possibly  be  any  other  conclusion  from  the 
structure  of  the  flowers  than  the  one  he  sug- 


ERRONEOUS  DEDUCTION.  197 

gested,  —  that  half  of  the  plants,  with  perfect 
flowers,  were  hermaphrodite,  and  the  other  half, 
with  rudimentary  stamens  and  anthers  and  no 
pollen,  were  female.  What  were  the  centuries 
of  study  on  plant  structure  worth,  if  it  was  not 
safe  to  accept  the  conviction  that  female  organs 
of  normal  size  are  present  for  the  bearing  of 
seed?  Darwin  said,  "The  most  acute  botanist, 
judging  only  from  structure,  would  never  have 
suspected  that  some  of  the  bushes  were  in  func- 
tion exclusively  males."1  As  in  the  previous 
example,  he  sought  to  verify  his  inference  by 
watching  the  fruit.  He  watched  thirteen  bushes, 
—  eight  females  and  five  "  hermaphrodites. " 
The  females  yielded  abundant  fruit,  and  a 
single  branch  two  or  three  feet  long  from  one 
of  them  yielded  more  than  any  one  whole  bush 
among  the  "hermaphrodites."  The  inference 
from  structure  was  almost  completely  reversed. 
The  species  seemed  to  be  practically  dioecious, 
with  the  stamens  aborted  in  the  females,  but 
the  pistils  apparently  normal  in  the  individuals 
that  were  almost  exclusively  male  in  function. 
He  might  here  again  have  rested  the  case,  and 
recorded  the  spindle-tree  as  dioecious. 

But  hear  him :   "  I  now  determined  to  observe 
more  carefully  during  successive  seasons   some 
1  Different  Forms  of  Flowers,  etc.,  pp.  287-292. 


198  THE  METHOD    OF  DARWIN. 

bushes  growing  in  another  place  about  a  mile 
distant."  He  did  so,  and  found  some  variability 
among  the  females  in  the  power  of  producing 
fruit  and  seed,  and  great  variability  in  the 
"hermaphrodites,"  the  latter  never  producing 
as  much  or  as  fine  seed  as  the  other.  At  this 
third  stage  it  was  clear  that  the  plants  of  the 
spindle-tree  are  neither  part  of  them  female  and 
part  hermaphrodite,  nor  part  of  them  female  and 
the  rest,  with  both  sets  of  organs,  practically 
male  in  function.  The  truth  lay  between  the 
two  extremes,  the  variations  in  the  one  or  other 
direction  depending  even  on  the  character  of 
the  season.  He  said,  "  This  case  appears  to  me 
very  interesting,  as  showing  how  gradually  an 
hermaphrodite  plant  maybe  converted  into  a 
dioecious  one."  The  final  result  of  the  long 
drawn  out  investigation  was  in  harmony  with 
his  general  doctrine  of  the  descent  of  species, 
and  is  an  illustration  of  some  of  the  best  evi- 
dence that  has  yet  been  adduced  in  its  support. 
To  him  it  was  interesting,  because  it  showed 
how  gradually  an  hermaphrodite  plant  may  be 
converted  into  a  dioecious  one.  To  the  student 
of  scientific  method  it  is  interesting  as  an 
example  of  how  an  investigation,  by  stopping 
short  of  exhaustion  of  the  field,  may  lead,  not 
only  to  imperfect,  but  to  false  conclusions. 


ERRONEOUS  DEDUCTION.  199 

In  the  work  on  the  Fertilization  of  Orchids 
Darwin's  sagacity  found  full  play  in  the  inter- 
pretation of  the  structure  of  orchids  in  ac- 
cordance with  the  principle  of  adaptation  for 
cross-fertilization.  Confident  that  all  the  mar- 
vellously complex  structures  of  orchidaceous 
flowers  were  adaptations  for  cross-fertilization, 
he  regarded  each  case  only  as  presenting  a 
question  with  regard  to  the  particular  mode  of 
the  mutual  action  of  the  insect  and  the  flower 
organs.  He  was  frequently  able  to  predict  this 
mutual  action  in  the  case  of  particular  species. 
He  never  for  a  moment  lost  faith  in  the  prin- 
ciple; but  in  at  least  one  case  he  went  wrong  in 
his  interpretation  by  not  applying  the  principle 
rigorously  enough. 

In  the  Cypripediumt  or  Lady's  Slipper,  there 
are  two  small  orifices  near  the  anthers  and  one 
large  opening  in  the  labellum  or  odd  petal.1 
After  discussing  the  structure  of  the  flower  he 
said,  "  Formerly  I  supposed  that  insects  alighted 
on  the  labellum  and  inserted  their  proboscides 
through  either  of  the  small  orifices  close  to  the 
anthers  "  to  extract  the  nectar.  This  inference 
seemed  plausible  enough;  the  small  orifices 
seemed  well  adapted  to  accomplish  cross-fertil- 
ization because  they  were  close  to  the  anthers, 

1  Fertilization  of  Orchids,  pp.  229-231. 


200  THE  METHOD   OF  DARWIN. 

and  an  insect  would  inevitably  remove  the  pol- 
linia  if  it  inserted  its  head  into  one  of  them. 
But  he  had  not  clung  closely  enough  to  his 
general  principle.  Darwin  himself  accepted 
the  correction  of  his  inference  as  follows  : 
"Delpino,  with  much  sagacity,  foresaw  that 
some  insect  would  be  discovered  "  to  remove 
the  pollinia  by  entering  the  labellum  by  its 
large  opening,  and  crawling  out  through  one  of 
the  orifices  near  the  anthers;  "for  he  argued 
that  if  an  insect  were  to  insert  its  proboscis,  as  I 
fhad  supposed,  from  the  outside  through  one  of 
the  small  orifices,  the  stigma  would  be  liable 
to  be  fertilized  by  the  plant's  own  pollen;  and 
in  this  he  did  not  believe,  from  having  con- 
fidence in  what  I  have  often  insisted  on,  — 
namely,  that  all  the  contrivances  for  fertiliza- 
tion are  arranged  so  that  the  stigma  shall 
receive  pollen  from  a  distinct  flower  or  plant. 
But  these  speculations  are  now  superfluous ;  for, 
owing  to  the  admirable  observations  of  Dr.  H. 
Miiller,  we  know  that  Cypripedium  calceolus,  in 
a  state  of  nature,  is  fertilized  in  the  manner 
just  described." 

Darwin's  error  in  this  case  consisted  in  not 
considering  one  of  the  important  elements  of 
his  principle  of  adaptation  for  cross-fertiliza- 
tion, —  namely,  the  importance,  not  only  of 


ERRONEOUS  DEDUCTION.  2O1 

favoring  cross-fertilization,  but  of  protecting 
the  stigma  against  self-fertilization.  When  this 
is  considered,  the  direction  in  which  the  insect 
entered  would  appear  much  more  important 
than  comparative  ease  in  getting  at  the  nectar 
or  proximity  of  the  orifices  to  the  stamens. 

One  of  the  most  remarkable  cases  of  over- 
sight in  Darwin's  work  occurred  in  connection 
with  the  Venus'  Fly-trap.  This  plant  is  re- 
stricted to  a  very  narrowly  limited  locality  in 
North  Carolina,  and  its  organs  are  highly  differ- 
entiated for  the  purpose  of  catching  insects. 
Darwin  could  not  reconcile  the  very  limited 
range  of  so  highly  specialized  an  insectivorous 
plant  with  his  general  belief  that  the  best 
adapted  plants  and  animals  spread  over  the 
earth  and  survive.  In  his  discussion  of  Dioncea, 
(Venus'  Fly-trap),  he  said,  "  It  is  a  strange  fact 
that  Dion&a,  which  is  one  of  the  most  beauti- 
fully adapted  plants  in  the  vegetable  kingdom, 
should  apparently  be  on  the  high  road  to  ex- 
tinction. This  is  all  the  more  strange  as  the 
organs  of  Dioncea  are  more  highly  differentiated 
than  those  of  Drosera."  l 

All  the  studi^  of  forty  years  had  borne  out 
the  conviction  that  the  adaptations  of  animals 
and  plants  to  their  environment  were  the  result 

1  Insectivorous  Plants,  p.  358. 


202  THE  METHOD   OF  DARWIN. 

of  natural  selection  acting  upon  variations,  and 
preserving  those  that  conduced  to  the  preserva- 
tion of  the  species.  Following  this  principle, 
it  seemed  almost  self-evident  that  the  more 
highly  specialized  the  organs  of  a  plant  or 
animal  are,  and  the  more  minutely  adapted  to 
its  surroundings  and  mode  of  life  they  become, 
the  more  certain  would  the  species  be  of  con- 
tinued existence  and  of  success  in  the  race  of 
life.  He  never,  so  far  as  I  know,  recognized 
the  inevitable  consequence  of  extreme  special- 
ization. Had  he  pursued  the  deduction  to  the 
end,  he  must  have  recognized  the  fact  that  a 
high  degree  of  specialization  for  a  particular 
mode  of  life  is  the  mess  of  pottage  for 
which  the  birthright  of  the  species  has  been 
surrendered. 

Darwin  was  familiar  with,  and  recognized 
the  value  of  Agassiz's  generalization  that  the 
progenitors  of  the  greater  animal  and  plant 
groups  have  been  generalized  forms.  He  him- 
self deduced  from  his  general  theory  the  prin- 
ciple that  the  species  of  the  larger  genera, 
the  wide  ranging,  much  diffused,  and  common 
species  vary  most,  that  they  are  more  closely 
related  to  each  other,  and  that  in  this  respect 
they  more  nearly  resemble  varieties  than  do  the 
species  of  the  smaller  genera  with  restricted 


ERRONEOUS  DEDUCTION.  2O3 

distribution.1  It  is  clear  that  the  wide  range 
of  a  species,  instead  of  depending  on  a  high 
degree  of  specialization  for  any  one  environ- 
ment, depends  rather  on  the  absence  of  it.  He 
clearly  recognized  and  pointed  out  the  danger 
of  extinction  to  a  species  of  limited  range,  but 
nowhere  recognized  explicitly  the  connection, 
on  the  one  hand,  between  a  high  degree  of 
specialization  for  a  particular  environment  or 
mode  of  life  and  restriction  of  the  species  to 
that  particular  environment,  or  the  relation,  on 
the  other  hand,  between  wide  range  over  the 
earth  and  generalized  characters  which  give 
some  general  advantage  that  would  be  useful 
under  all  or  nearly  all  circumstances  into  which 
the  species  might  be  thrown. 

He  said:  "If  we  ask  ourselves  why  this  or 
that  species  is  rare,  we  answer  that  something 
is  unfavorable  in  its  conditions  of  life;  but 
what  that  something  is  we  can  hardly  ever  tell." 
He  insisted  that  the  improved  and  modified 
forms  would  crowd  out  and  exterminate  the  less 
well  adapted  forms;  but  did  not  hit  upon  the 
truth  that,  the  more  beautifully  adapted  a 
species  is  to  a  definite  locality  and  set  of  con- 
ditions, the  less  it  is  adapted  to  enter  into  a 
general  competition  for  the  possession  of  the 

1  Origin  of  Species,  pp.  42-44. 


204  THE  METHOD   OF  DARWIN. 

earth.  Both  Agassiz's  empirical  generaliza- 
tion that  the  progenitors  of  the  principal  plant 
and  animal  groups  were  generalized  forms, 
and  the  deductive  consequences  of  Darwin's 
own  theory  of  natural  selection  would  indicate 
that  the  highly  differentiated  forms  are  forever 
handicapped.  They  have  a  present  advantage 
in  having  intensified  their  adaptation  to  a  cer- 
tain environment;  but  in  the  long  run  they 
are  doomed  because  they  have  lost  the  power 
of  adaptability  to  new  conditions  in  direct 
proportion  to  their  present  gain. 

It  is  curious  that  Darwin  had  before  him, 
and  mentioned  in  the  same  sentence,  a  case  of 
each  kind:  Dioncea,  with  its  extreme  adapta- 
tions for  insect  catching,  with  only  a  single 
species  in  the  genus,  restricted  to  a  very  small 
locality,  on  the  verge  of  extinction ;  and  Drostra, 
with  the  same  general  advantage  of  catching 
insects,  but  with  no  extreme  adaptations,  with 
a  vast  number  of  species  in  the  genus,  dis- 
tributed all  but  everywhere  over  the  earth.  It 
is  as  great  a  surprise  that  he  did  not  see  the 
connection  between  extreme  specialization  and 
extinction  on  the  one  hand,  and  general  advan- 
tage and  wide  distribution  on  the  other,  as  he 
says  it  was  to  him  that  Dioncza  is  on  the  verge 
of  extinction.  It  is  clearly  inferable  from 


ERRONEOUS  DEDUCTION.  205 

these  cases  and  the  general  principle  of  natural 
selection  that  Nature  has  pronounced  the  sen- 
tence of  death  upon  highly  specialized  forms; 
that  they  have  passed  out  of  the  royal  line  of 
descent  for  a  special  advantage;  that  if  they 
vary  at  all  it  can  only  be  within  the  restricted 
lines  along  which  they  have  already  gone  so 
far;  and  that  the  birthright  belongs  to  forms 
which  have  some  general  advantage,  but  are 
not  hampered  by  special  adaptations.  These 
spread  over  the  earth  and  from  them  branch 
off  the  numerous  closely  related  and  variable 
species  that  occupy  all  the  environments  of  the 
earth.  Thus  Drosera,  with  its  power  to  catch 
insects,  is  yet  a  plant  like  others,  and  has  gone 
forth  with  its  advantage  to  possess  the  earth. 
Vitis,  with  its  climbing  power,  has  scattered 
its  species  by  the  hundred  over  the  earth.  It 
is  the  modification  that  opens  up  to  the  species 
a  large  area,  which  makes  it  possible  for  the 
species  to  send  out  its  kind  into  the  whole 
earth  to  be  everywhere  modified  by  local  influ- 
ences. 

The  principle  that  has  been  discussed  is  now 
one  of  the  well  understood  corollaries  of  the 
principle  of  natural  selection.  I  do  not  know 
who  first  called  attention  to  it.  It  has  prob- 
ably occurred  to  many  minds  independently. 


206  THE  METHOD   OF  DARWIN. 

Darwin,  so  far  as  I  know,  never  recognized  it. 
The  earliest  clear  statement  of  it  that  I  have 
seen  is  by  Prof.  Joseph  Le  Conte  in  an  article 
on  "Instinct  and  Intelligence,"  1  published  in 
1875,  only  a  few  months  after  the  publication 
of  Darwin's  "  Insectivorous  Plants. "  He  said, 
"Instinct,  therefore,  is  accumulated  experi- 
ence, or  knowledge  of  many  generations  fixed 
M  permanently  and  petrified  in  brain-structure. 
All  such  petrifaction  arrests  development,  be- 
cause unadaptable  to  new  conditions.  They  are 
J  £ound,  therefore,  only  in  classes  and  families 
J^jJjjy^  widely  differentiated  from  the  main  stem  of  evolu- 
tion, from  the  lowest  animals  to  man.  Instincts 
are,  indeed,  the  flower  and  fruit  at  the  end  of 
these  widely  differentiated  branches,  but  flower- 
ing and  fruiting  arrest  onward  growth."  In 
1877,  Marsh,  in  a  discussion  of  the  suilline 
type,  stated  the  true  principle  when  he  said 
that  ambitious  offshoots  have  perished,  while 
the  generalized  or  rather  unspecialized  forms 
continue  the  line  of  life  with  true  suilline 
stubbornness.2 

1  Popular  Science  Monthly,  October,  1875,  P-  664. 

2  American  Journal  of  Science,  XIV.  (III.),  pp.  362-364. 


XIV. 

GENERAL  DISCUSSIONS. 

DARWIN'S  general  discussions  of  the 
various  subjects  that  he  worked  out  in 
such  minute  detail  are  models  both  of  clearness 
and  of  exhaustiveness.  The  reasoning  of  the 
second  volume  of  the  "Variation  of  Animals 
and  Plants  under  Domestication  "  is  long  sus- 
tained and  characterized  by  the  use  of  enor- 
mous numbers  of  facts.  The  latter  is  one  of 
the  chief  characteristics  of  his  reasoning.  He 
was  frequently  compelled,  as  in  the  case  of 
the  "Origin  of  Species,"  by  the  limits  within 
which  he  was  obliged  to  condense  his  materials, 
to  substitute  general  statements  for  long  series 
of  facts.  The  generalizations  were  condensa- 
tions of  the  detailed  facts,  which  were  too 
bulky  for  his  pages.  This  is  the  reason  why  his 
discussions  leave  the  impression  of  an  almost 
infinite  reserve  of  evidence;  and  rightly  seem 
to  convey  much  more  to  the  reader  than  is 
actually  written  on  the  page. 

Among   Darwin's   many   exhaustive   discus- 


208  THE  METHOD   OF  DARWIN. 

sions  of  the  materials  which  he  had  collected, 
some  of  the  most  striking  are  his  chapters  on 
pigeons,1  in  which  he  considered  the  variation 
of  breeds,  individual  differences,  and  skeletal 
differences;  the  discussion  in  the  second  volume 
of  the  "Variation  of  Animals  and  Plants  under 
Domestication  "  ;  the  exhaustive  analysis  of  his 
materials  in  reaching  his  general  conclusions 
on  the  effects  of  cross- and  self-fertilization; 
and  his  repeated  discussions  of  the  cement 
glands  of  Cirripedia,  and  of  the  parasitic  and 
complemental  males  on  the  hermaphrodite 
"females,"  and  the  reasons  for  regarding  them 
as  such  rather  than  as  independent  forms.2 
But  the  general  discussion  that  is  typical  both 
from  the  general  interest  of  the  subject  and 
the  compactness  and  symmetry  of  the  argument 
is  the  work  on  the  "  Origin  of  Species.""  It 
would  be  impossible  to  analyze  such  a  far- 
reaching  argument  without  restating  it.  Doubt- 
less criticisms  could  be  made  against  the 
arrangement  of  materials  and  the  order  of  dis- 
cussion, and  against  the  nature  of  the  evidence 
adduced.  It  is  not  intended  here  to  raise  the 

1  Variation  of  Animals  and  Plants  under  Domestication, 
Vol.  I.  pp.  I37-235- 

2  Monograph  of  Cirripedia,  Vol.  I.  pp.  38,  180-293,  Vol.  II. 
pp.  23-30,  151. 


GENERAL  DISCUSSIONS.  2  09 

question  whether  his  argument  as  a  whole  is 
sound  and  carries  conviction  with  it,  but  simply 
to  repeat  that,  considering  the  materials  that 
Darwin  had  to  work  with  and  the  difficulties 
under  which  he  labored,  the  argument  is 
finished,  and  will  always  serve  as  a  type  of 
probable  proof. 

Darwin  himself  has  given  the  reasons  for 
this  state  of  his  great  discussion.  He  has  com- 
plained that  he  must  be  a  very  slow  thinker; 
and  doubtless  the  truth  in  this  complaint 
accounts  for  the  fact  that  his  thinking  was 
always  so  thorough.  He  also  bewailed  the  fact 
that  he  experienced  great  difficulty  in  writing.1 
He  felt  that  he  had  great  ability  to  get  things 
wrong  end  foremost  in  his  expression,  and  said 
that  he  spent  a  great  deal  of  time  in  arranging 
the  matter  in  his  larger  works.  From  one 
point  of  view  it  is  a  paradox  that  such  a  man 
should  accomplish  so  much  that  has  proved 
of  permanent  value.  But  the  lack  of  natural 
felicity  of  expression  and  inability  to  think 
rapidly,  together  with  his  persistence,  insured 
him  against  the  vice  of  saying  things  nimbly, 
and  furnished  the  guaranty  that  whatever  he 
did  would  be  thorough.  It  is  safe  to  say  that 
a  far  larger  proportion  of  false  and  inaccurate 

1  Lifs  and  Letters,  Vol.  I.  p.  80. 
14 


210  THE  METHOD   OF  DARWIN. 

statements  and  arguments  with  fatal  flaws  in 
them  are  made  by  writers  who  express  them- 
selves easily,  than  by  writers  whose  rhetorical 
inability    compels    them    to    be    painstaking. 
When  the  expression  is  laboriously  evolved  by 
an  intellect  that  is  otherwise  strong,  the  thought 
comes  during  the  process  to  be  regarded  from 
more  points  of  view.     In  details  there  is  greater 
assurance  of  accuracy,  and  the  proper  relative 
importance  is  more  likely  to  be  assigned  to  the 
different  phases  of  the  truth.     Given  two  intel- 
lects equally  conscientious,  the  slower  moving 
and  more  deliberate  one  will  always  hit  upon 
more  phases  of  the  truth  than  the  quicker  one. 
Darwin  attributed  the  success  of  the  "  Origin 
of  Species"  to  the  way  in  which  it  was  devel- 
oped.     Only  when  he  had  spent  a  number  of 
years  in  investigation,  after  he  had  got  hold  of 
the  theory  of  natural  selection,  did   he  allow 
himself,  in   1842,  to  draw  up  a  brief  thirty-five 
page  sketch  of  it.      In   1844  he  wrote  a  larger 
sketch  of  two  hundred  and  thirty  pages.     Then 
followed  years  of  laborious  investigation,  the 
vast  results  of  which  were  cast  into  an  abstract 
which,   if  it  had  been  published,    would  have 
been   a   very   large   work.      Upon   the   urgent 
advice   of   his  friends,   Lyell  and  Hooker,  he 
decided   not   to  delay  publication  any  longer; 


GENERAL   DISCUSSIONS.  211 

but  the  large  work  was  not  in  condition  for 
publication.  An  abstract  of  it  was  made;  and 
this  abstract  was  the  "  Origin  of  Species."  He 
had  written  two  condensed  sketches,  and  finally 
abstracted  a  much  larger  manuscript  which  was 
itself  an  abstract.1  Probably  no  work  was  ever 
better  tempered  and  tested  before  it  reached 
the  public  eye. 

1  Life  and  Letters,  Vol.  I.  p.  70. 


XV. 

LOGICAL  HISTORY  OF  THE   PRINCIPLE   OF 
NATURAL  SELECTION. 

T^vARWIN'S  own  views  of  method,  his  treat- 
•L ^  ment  of  evidence,  and  some  of  the 
various  logical  processes  which  he  employed  in 
his  investigations,  having  been  discussed  and 
illustrated  by  examples  from  his  many  works, 
it  now  remains  necessary  to  trace  the  logical 
history  and  examine  the  present  logical  status 
of  the  principle  of  natural  selection,  which  gave 
inspiration  to  and  lay  at  the  basis  of  his  life- 
work.  The  theory  of  natural  selection  has  per- 
meated and  colored  modern  thought  more  deeply 
than  perhaps  any  other  scientific  theory,  and 
this  fact  alone  makes  the  study  of  its  logical 
history  extremely  interesting. 

The  scientific  influences,  both  in  the  form 
of  teachers  and  of  books,  to  which  Darwin  was 
exposed  during  and  after  his  university  life, 
were  opposed  to  the  already  well  known  doc- 
trine of  the  descent  of  species.  Various  reasons 
have  been  assigned  for  the  failure  of  the  doc- 


PRINCIPLE   OF  NATURAL  SELECTION.      21$ 

trine  to  impress  itself  upon  scientific  men. 
The  difficulty  did  not  lie  in  the  circumstance 
that  the  facts  of  botany  and  zoology  were 
opposed  to  it;  for  it  first  took  its  rise  out  of 
them.  The  affinities  of  species  and  of  the 
higher  groups,  and  the  facts  of  embryology, 
distribution,  and  palaeontology  by  themselves 
were  sufficient  to  force  the  conviction  that 
species  are  derived,  and  the  doctrine  would 
doubtless  have  won  its  way  at  once  had  it  not 
had  to  make  head  against  the  imported  belief 
in  creation.  Had  the  doctrines  of  descent  and 
creation  been  for  the  first  time  presented  to  the 
scientific  mind  as  alternative  beliefs,  there  can 
be  no  doubt  that  the  former  would  have  been 
chosen  as  the  true  explanation  of  the  facts, 
even  though  no  force  capable  of  producing  the 
effects  had  been  assigned.  The  cause  would 
still  remain  to  be  investigated,  while  the  facts 
would  be  brought  together  under  a  single  point 
of  view.  With  the  adoption  of  creation  as  an 
explanation,  an  efficient  cause  is  provided,  but 
the  facts  remain  worthless  either  to  prove  or 
to  disprove  the  doctrine.  It  is  not  enough  that 
a  cause  should  be  capable  of  producing  given 
effects,  but  it  should  produce  the  given  effects 
and  be  incapable  of  producing  any  other  set 
of  effects.  In  short,  by  the  former  view  the 


214  THE  METHOD   OF  DARWIN. 

special  character  of  the  facts  is  accounted  for, 
but  from  the  latter  it  is  impossible  to  deduce 
the  specific  character  of  any  phenomena.  Any 
other  set  of  facts  exhibiting  a  plan  or  purpose 
of  any  kind  could  be  deduced  with  equal  ease 
from  the  doctrine  of  creation. 

When  Darwin  started  on  the  Beagle  voyage 
he  was  orthodox  on  the  question  of  the  ori- 
gin of  species.  As  he  travelled,  and  as  his 
knowledge  of  zoology  and  palaeontology  became 
wider  and  deeper,  the  doctrine  of  descent  began 
to  take  hold  of  him.  The  relation  of  the  liv- 
ing animals  to  the  fossil  species  in  South 
America,  the  manner  in  which  closely  allied 
animals  replaced  one  another  as  he  proceeded 
southward  over  the  continent,  the  South  Amer- 
ican character  of  the  productions  of  the  Gala- 
pagos archipelago,  and  especially  the  slight 
but  distinct  differences  of  the  flora  and  fauna 
on  neighboring  islands  of  the  archipelago,  im- 
pressed him  so  strongly  with  the  peculiar  char- 
acter of  the  facts  and  the  necessity  of  a  definite 
mode  of  origin  that  he  began  to  see  the  differ- 
ence in  the  logical  characters  of  the  doctrines  of 
creation  and  descent1  The  facts  were  better 
explained  by  the  latter  than  by  the  former;  and 
he  connected  them  at  least  tentatively  with  the 

1  Life  and  Letters,  Vol.  I.  p.  67 ;  Origin  of  Species,  p.  2. 


PRINCIPLE  OF  NATURAL  SELECTION.      21$ 

old  doctrine  of  descent.  But  as  he  himself 
remarked,  a  naturalist  might  be  convinced  by 
affinities,  embryology,  distribution,  etc.,  that 
species  are  derived  by  descent;  but  the  conclu- 
sion, though  correct,  would  be  unsatisfactory 
until  it  was  shown  how  it  was  brought  about. 

Lamarck  had  assigned  causes  for  the  modifi- 
cation of  species ;  but  Darwin  insisted  positively 
that  he  derived  no  help  whatever  from  him. 
Logically,  the  failure  of  the  former's  doctrines 
to  win  over  the  scientific  world  was  due  to  his 
connecting  the  doctrine  of  descent,  which  had 
strong  evidence  in  its  favor,  with  hypothetical 
causes  which  he  did  not  subject  to  rigid  de- 
ductive tests  in  explanation  of  the  facts.  What 
kept  Darwin  longest  orthodox  were  the  facts 
of  adaptation.  No  cause  could  be  considered 
adequate  which  did  not  account  for  the  exqui- 
site adaptation  to  environment  found  through- 
out the  animal  and  plant  kingdoms.  As  he 
said  himself,  "  I  had  always  been  much  struck 
by  such  adaptation,  and  until  these  could  be 
explained  it  seemed  to  me  almost  useless  to 
endeavor  to  prove  by  indirect  evidence  that 
species  have  been  modified."1  He  distin- 
guished clearly  in  his  own  mind  between  the 
two  propositions  which  he  undertook  to  prove 

1  Life  and  Letters,  Vol.  I.  p.  67. 


2l6  THE  METHOD    OF  DARWIN. 

in  the  "  Origin  of  Species  " :  first,  that  species 
are  derived  by  modification  from  other  species, 
and,  secondly,  that  Natural  Selection  is  the 
chief  cause  of  this  modification.  He  did  not 
originate  the  former;  the  logical  relation  of  his 
work  to  it  is  deductive,  and  largely  took  the 
form  of  an  answer  to  the  question,  Do  the  facts 
of  Nature  harmonize  with  the  hypothesis? 

The  cause  of  the  modification  of  species  could 
not  even  be  raised  as  a  question  until  the  fact 
of  modification  had  been  accepted,  at  least  ten- 
tatively. He  recognized  the  cause  of  specific 
modification  as  a  problem  to  be  solved,  a  prin- 
ciple to  be  discovered  by  induction  from  the 
effects  it  produced  in  the  form  of  adaptations. 
To  quote  his  own  words:  "My  first  note-book 
was  opened  in  July,  1837.  I  worked  on  true 
Baconian  principles,  and  without  any  theory 
collected  facts  on  a  wholesale  scale,  more 
especially  with  respect  to  domesticated  pro- 
ductions," etc.1  He  had  rejected  Lamarck's 
suggestions,  and  it  would  seem  impossible  to 
imagine  a  more  interesting  or  more  purely 
inductive  problem  than  that  which  presented 
itself  to  him.  There  was  for  him  no  clew  to 
the  cause  which  he  wished  to  discover  except 
in  the  vast  wealth  of  material  which  he  regarded 

i  Life*nd  Letters,  Vol.  I.  p.  68. 


PRINCIPLE   OF  NATURAL   SELECTION,      21  / 

as  its  effects.  To  have  discovered  the  cause 
by  an  analysis  of  the  effects  would  indeed  have 
been  a  triumphant  inductive  discovery. 

He  selected  wisely  the  material  on  which 
to  concentrate  the  investigation ;  he  said :  "  I 
soon  perceived  that  selection  was  the  keystone 
of  man's  success  in  making  useful  races  of 
animals  and  plants.  But  how  selection  could 
be  applied  to  organisms  living  in  a  state  of 
nature  remained  for  some  time  a  mystery  to 
me."  In  his  study  of  domestic  races  he  ob- 
served both  the  effects  (races)  and  the  cause 
(selection),  and  did  not,  except  perhaps  in 
details,  reason  deductively  from  the  cause  to 
discover  the  effect,  or  inductively  from  the 
effects  to  discover  the  cause.  The  effort  to 
extend  the  principle  of  selection  by  induction 
to  animals  and  plants  in  a  state  of  nature  failed 
because  it  was  impossible  to  see  how  the  prin- 
ciple could  be  applied.  The  inductive  problem 
was  apparently  as  far  from  solution  as  at  the  be- 
ginning; he  was  still  groping  in  the  dark.  It 
would  be  a  bootless  speculation  to  try  to  answer 
the  question  whether  Darwin  could  ever  have 
solved  by  a  study  of  adaptations  the  problem 
which  he  set  for  himself.  It  would  be  hardy 
to  hold  that  a  man  with  Darwin's  intellectual 
and  moral  resources,  with  his  clear  conception 


2l8  THE  METHOD   OF  DARWIN. 

of  the  problem  and  the  data  from  which  it  was 
to  be  solved,  could  not  have  derived  the  cause 
from  an  analysis  of  the  effects ;  and  yet  very 
few  problems  like  this  were  ever  solved  by 
pure  induction.  It  may  be  possible  to  infer 
the  nature  of  a  cause  from  the  nature  of  the 
effects,  but  nearly  always  observers  manage  to 
catch  a  glimpse  of  the  cause  at  work.  Then, 
by  a  generalization,  the  cause  is  extended  to  all 
the  other  effects  of  the  same  kind. 

In  October,  1838,  at  the  end  of  fifteen 
months  of  work  on  Baconian  principles,  with- 
out any  theory,  he  read  Malthus  on  Population 
for  amusement.1  There  had  been  much  dis- 
cussion in  the  eighteenth  century  concerning 
the  vice  and  misery  in  human  society.  It 
was  quite  commonly  believed  that  they  were 
due  to  the  organization  of  society,  and  tha-t 
they  could  be  eliminated  by  reorganization  of 
society  according  to  some  ideal.  The  father 
of  Malthus  shared  this  view;  but  the  son,  in 
discussion  with  him,  took  the  position  that,  no 
matter  how  society  might  be  organized,  vice  and 
misery  would  follow  inevitably  from  the  fact 
that  the  human  race  naturally  increases  more 
rapidly  than  the  means  of  subsistence.  This 
notion  was  finally  developed  into  the  "  Prin- 

1  Life  and  Letters,  Vol.  I.  p.  68. 


PRINCIPLE   OF  NATURAL  SELECTION.      2IQ 

ciple  of  Population,"  which  fell  into  Darwin's 
hands.  The  purpose  of  Malthus  in  this  work 
was  to  investigate  the  causes  that  had  hitherto 
impeded  the  progress  of  mankind  toward  hap- 
piness. After  establishing  the  principle  that 
population  has  a  tendency  to  increase  in  geo- 
metrical ratio,  while  the  food  supply  can  at  best 
increase  only  in  arithmetical  ratio,  he  pointed 
out  that  the  ultimate  check  to  the  increase  of 
population  is  lack  of  food;  and  that  all  the 
immediate  checks  could  be  included  under  three 
heads,  moral  restraint,  vice,  and  misery,  — and 
urged  moral  restraint  as  a  check  to  population, 
because  by  it  alone  could  vice  and  misery  be 
driven  out  of  the  world.1 

Malthus  stated  with  perfect  clearness  "the 
constant  tendency  in  all  animated  life  to  in- 
crease beyond  the  nourishment  prepared  for 
it,"  and  the  consequent  struggle  for  existence; 
and  insisted  that  in  every  country,  speaking  of 
the  human  family,  some  of  the  checks  to  popu- 
lation are,  with  more  or  less  force,  in  constant 
operation.2  He  recognized  both  artificial  and 
natural  selection  as  results  of  the  struggle  for 
existence.  In  the  chapter  on  the  "  Checks  to 
Population  among  the  American  Indians,"  he 
said:  — 

1  Malthus,  Principle  of  Population  (gth  edition),  pp.  1-8. 

2  Ibid.,  p.  9. 


22O  THE  METHOD   OF  DARWIN. 

"As  the  parents  are  frequently  exposed  to 
want  themselves,  the  difficulty  of  supporting 
their  children  becomes  at  times  so  great  that 
they  are  reduced  to  the  necessity  of  abandon- 
ing or  destroying  them.  Deformed  children 
are  very  generally  exposed ;  and  among  some 
of  the  tribes  in  South  America,  the  children  of 
mothers  who  do  not  bear  their  labors  well  ex- 
perience a  similar  fate  from  a  fear  that  the  off- 
spring may  inherit  the  weakness  of  its  parent. 

"To  causes  of  this  nature  we  must  ascribe 
the  remarkable  exemption  of  the  Americans 
from  deformities  of  make.  Even  when  a  mother 
endeavors  to  rear  all  her  children  without  dis- 
tinction, such  a  proportion  of  the  whole  number 
perishes  under  the  rigorous  treatment  which 
must  be  their  lot  in  the  savage  state,  that  prob- 
ably none  of  those  who  labor  under  any  original 
weakness  or  infirmity  can  attain  the  age  of 
manhood.  If  they  be  not  cut  off  as  soon  as 
they  are  born,  they  cannot  long  protract  their 
lives  under  the  severe  discipline  that  awaits 
them.  In  the  Spanish  provinces,  where  the 
Indians  do  not  lead  so  laborious  a  life,  and 
are  prevented  from  destroying  their  children, 
great  numbers  of  them  are  deformed,  dwarfish, 
mutilated,  blind,  and  deaf."1 

1  Principle  of  Population,  Chap.  IV.  pp.  20,  21. 


PRINCIPLE   OF  NATURAL   SELECTION.     221 

It  seems  almost  astounding  that  Malthus  did 
not  recognize  the  importance  of  this  principle 
of  natural  selection  based  on  the  struggle  for 
existence  and  develop  it  deductively.  Had  he 
worked  out  the  principle  of  the  survival  of  the 
fittest  among  human  beings  after  he  so  clearly 
recognized  it,  it  would  have  borne  rich  fruit 
for  the  happiness  doctrine.  It  would  have  re- 
moved much  of  the  gloom  from  his  principle  of 
population,  by  showing  that  much  permanent 
good  —  much  more,  in  fact,  than  from  moral 
restraint  —  arises  from  the  struggle  for  exist- 
ence by  its  preserving  those  best  fitted  to  enjoy 
life.  From  another  point  of  view,  the  fact  that 
he  did  not  develop  the  principle  of  natural 
selection,  at  least  within  the  human  race,  after 
he  had  so  plainly  recognized  both  its  action 
and  its  effects,  is  not  even  remarkable.  When 
one  has  once  made  a  study  of  the  deductive 
powers  of  such  a  man  as  Darwin,  and  finds  that 
with  his  great  logical  strength  he  sometimes 
failed  altogether,  and  was  often  very  long  in 
reaching  deductively  the  consequences  of  his 
theory,  it  is  not  to  be  wondered  at  that  Malthus 
did  not  grasp  one  of  the  most  important  features 
of  his  principle. 

Since  he  failed  to  apply  the  principle  of 
natural  selection  within  the  human  species, 


222  THE  METHOD  OF  DARWIN. 

after  he  had  himself  stated  clearly  its  action 
and  results,  there  is  no  cause  for  wonder  at  his 
not  applying  it  to  the  derivation  of  species. 
In  fact,  there  was  a  good  reason  why  he  should 
not  do  the  latter.  He  expressed  himself  vigor- 
ously in  opposition  to  the  belief  that  a  species 
can  vary  by  an  indefinite  amount  in  any  given 
direction;  and  denied  Condorcet's  theory  of 
the  indefinite  perfectibility  of  the  human  race.1 
He  admitted  only  a  limited  amount  of  varia- 
tion within  a  species.  Malthus.  had  worked  out 
the  struggle  for  existence  and  recognized  its 
selective  action,  at  least  within  a  limited  range; 
and  as  a  natural  theologian  he  must  have  been 
acquainted  with  numerous  adaptations.  But 
he  did  not  connect  them  as  cause  and  effect, 
because  he  did  not  admit  general  variability 
of  species,  without  which  there  could  be  no 
material  for  the  cause  to  act  upon. 

When  the  work  of  Malthus  fell  into  Darwin's 
hands,  the  latter  was  in  possession  of  the  doc- 
trine of  descent,  and  many  facts  in  harmony 
with  it,  and  hence  the  convi.ction  in  his  mind 
that  there  was  an  efficient  cause  for  these  facts. 
The  adaptations  found  in  nature  seemed  the 
most  difficult  facts  to  explain  under  the  theory 
of  descent,  and  Darwin  was  already  widely 

1  Principle  of  Population,  p   270. 


PRINCIPLE    OF  NATURAL  SELECTION.    22$ 

acquainted  with  these.  He  had  recognized,  as 
did  others  before  him,  specific  variations  as 
material  out  of  which  new  species  must  be 
made,  and  had  gone  to  work  systematically  to 
study  variations,  especially  in  domestic  pigeons, 
the  most  favorable  group  of  animals  that  could 
have  been  selected ;  and  had  recognized  selec- 
tion as  the  key  to  man's  success  in  the  produc- 
tion of  races.  His  study  hitherto  had  helped 
him  to  a  wider  knowledge  and  clearer  notion 
of  adaptations,  —the  effects  that  had  to  be 
explained;  and  a  better  understanding  of  the 
nature  and  range  of  variations,  — the  materials 
upon  which  the  cause  must  have  acted  to  pro- 
duce the  effects.  And  in  the  case  of  domestic 
productions  he  was  in  full  possession  of  the 
cause,  the  materials  on  which  the  cause  acted, 
and  the  results,  in  selection,  variations,  and 
races.  As  he  himself  said,  he  had  been  fully  pre- 
pared by  his  long  study  of  the  habits  of  animals 
to  appreciate  the  struggle  for  existence  which 
everywhere  goes  on.  "  It  at  once  struck  me,"  he 
said,  "that  under  these  circumstances  favorable 
variations  would  be  preserved  and  unfavorable 
ones  destroyed.  The  result  of  this  would  be 
the  formation  of  new  species.  Here,  then,  I  had 
at  last  got  a  theory  by  which  to  work."  1 

1  Life  and  Letters,  Vol.  I.  p.  68. 


224  THE  METHOD   OF  DARWIN. 

It  is  not  recorded  at  what  point  in  his  read- 
ing of  Malthus  it  struck  him  that  the  struggle 
for  existence,  by  working  upon  variations,  would 
produce  new  species.  Probably  with  his  mind 
so  thoroughly  imbued  with  the  subject  that 
everything  he  read  was  made  to  bear  upon  the 
problem  he  almost  instantaneously  caught  the 
significance  of  the  principle.  Malthus  himself 
stated  the  principle  clearly  in  the  first  few 
pages  of  the  book,  and  already  on  pages  21,  22, 
of  the  ninth  edition  stated  its  action  and  effects 
upon  the  American  Indians.  The  significance 
of  these  details  is  great  from  the  logical  point 
of  view.  All  the  years  of  the  Beagle  voyage 
had  prepared  Darwin  to  appreciate  the  principle. 
The  time  since  his  return  had  strengthened  his 
belief  in  the  descent  of  species,  and  his  efforts 
to  reach  the  cause  of  modification  inductively 
had  brought  him  detailed  knowledge  of  both 
variations  and  adaptations.  What  he  had  not 
hitherto  been  able  to  discover  by  induction 
came  to  him  by  accident,  if  it  can  be  said  that 
anything  can  come  by  accident  to  a  mind  so 
much  on  the  alert  for  it.  It  would  have  been 
one  of  the  most  fascinating  chapters  in  scien- 
tific discovery  if  he  had  recorded  in  detail  the 
mental  activity  and  the  feelings  that  must  have 
flooded  him  from  the  moment  the  discovery  was 


PRINCIPLE   OF  NATURAL  SELECTION.    22$ 

made.  There  was  an  intellectual  explosion,  a 
flash  of  the  mind,  and  from  that  moment  his 
life-work  was  devoted  to  elaborating  the  conse- 
quences of  the  principle.  The  facts  which  he 
had  been  gathering  and  reflecting  upon  were 
explained  as  the  effects  of  the  cause  which 
Malthus  presented,  and  gathered  a  new  sig- 
nificance from  it. 

"  Here,  then,  I  had  at  last  got  a  theory  by 
which  to  work,"  he  said.  The  groping  was  at 
an  end.  His  future  work  was  outlined.  The 
confession  in  that  sentence  can  be  appreciated 
only  by  one  who  has  in  his  own  experience 
passed  from  the  mental  strain  and  perplexity 
of  a  purely  inductive  effort  to  the  solid  ground 
afforded  by  even  a  fairly  probable  hypothesis. 
Doubtless  his  work  was  thenceforth  many  times 
more  rapid  than  it  could  otherwise  have  been; 
for  with  so  vast  a  number  of  facts  to  be  con- 
sidered the  theory  itself  was  the  only  pathfinder. 
Only  after  the  discovery  of  the  principle  could 
the  work  of  gathering  up  and  classifying  known 
facts  and  of  searching  for  new  ones,  of  reducing 
exceptions  and  apparently  unexplainable  groups 
of  facts,  go  on  apace.  The  logical  process  by 
which  adaptations,  variations,  and  the  struggle 
for  existence  were  brought  together  into  the 
relation  of  cause  and  effect  was  deductive;  and 


226  THE  METHOD   OF  DARWIN. 

the  principle  of  Natural  Selection  still  de- 
pends for  its  logical  support  upon  that  power  of 
deductive  explanation  which  Darwin  recognized 
in  it  the  day  he  read  Malthus  on  Population. 
It  has  penetrated  every  field  of  thought,  but  in 
the  field  in  which  it  first  gathered  strength  it 
is  still  without  direct  demonstration.  It  has 
been  made  the  basis  for  countless  deductive 
operations,  but  it  leans  for  support  on  the  very 
structures  thus  erected.  Writing  to  Bentham, 
in  1863,  concerning  the  proofs  of  natural  selec- 
tion and  the  descent  of  species,  Darwin  said, 
"Belief  in  natural  selection  must  at  present  be 
grounded  entirely  on  general  considerations : 
(i)  on  its  being  a  vera  causa  from  the  struggle 
for  existence;  ...  (2)  from  the  analogy  of 
change  under  domestication  by  man's  selection; 
(3)  and  chiefly  from  this  view  connecting 
under  an  intelligible  point  of  view  a  host  of 
facts. " 1  To  Huxley  he  said,  in  December, 
1860,  "I  can  pretty  plainly  see  that,  if  my 
view  is  ever  to  be  generally  adopted,  it  will 
be  by  young  men  growing  up  and  replacing  the 
old  workers,  .  .  .  and  finding  out  that  they  can 
group  facts  and  search  out  new  lines  of  inves- 
tigation better  on  the  notion  of  descent  than  on 
that  of  creation."2 

1  Life  and  Letters,  Vol.  II.  p.  210.         2  Ibid.,  p.  147. 


PRINCIPLE    OF  NATURAL  SELECTION.       22  J 

From  a  logical  point  of  view  the  work  of  the 
last  thirty  years  in  the  various  fields  of  biology 
has  been  a  series  of  deductions  and  verifica- 
tions of  the  original  propositions  laid  down 
by  Darwin.  He  saw  from  the  beginning  that 
belief  in  his  theory  must  rest  on  general  con- 
siderations, the  chief  of  which  was  its  power 
to  facilitate  deductive  investigation;  and  there 
it  still  rests.  At  this  late  day  the  chief  apostle 
of  natural  selection  says  that  it  is  really  diffi- 
cult to  imagine  the  process  of  natural  selection 
in  its  details,  and  that  it  is  impossible  to  this 
day  to  demonstrate  it  in  any  one  point.1  It 
is  the  logical  relation  of  the  principle  to  the 
facts  that  makes  it  invaluable  in  modern 
thought.  The  whole  logical  history  of  Dar- 
win's principles  illustrates  what  Mill  said  of 
the  deductive  method.  "To  the  Deductive 
Method  thus  characterized  in  its  three  con- 
stituent parts,  Induction,  Ratiocination,  and 
Verification,  the  human  mind  is  indebted  for 
its  most  conspicuous  triumphs  in  the  investiga- 
tion of  nature.  To  it  we  owe  all  the  theories 
by  which  vast  and  complicated  phenomena  are 
embraced  under  a  few  simple  laws,  which,  con- 
sidered as  the  laws  of  these  great  phenomena, 

1  Weissmann,  Contemporary  Review,  September,  1893,  Vol. 
LXIV.  p.  322. 


228  THE  METHOD   OF  DARWIN. 

could  never  have  been  detected  by  their  direct 
study."  Writing  of  the  celestial  motions  as 
an  illustration,  he  continued,  "How  could  we 
ever  have  ascertained  the  combination  of  forces 
on  which  the  motions  of  the  earth  and  planets 
are  dependent  by  merely  comparing  the  orbits 
or  velocities  of  different  planets  or  the  different 
velocities  and  positions  of  the  same  planet  ?  " 

Darwin  himself  did  not  discover  the  cause  by 
the  direct  study  of  the  effects ;  but  his  efforts 
to  reach  a  cause  inductively  gave  him  such  an 
insight  into  variations  and  adaptations  that  he 
could  prosecute  vigorously  the  other  two  steps, 
deduction  and  verification,  when  once  the  cause 
was  given.  What  Mill  said  of  celestial  motions 
could  be  almost  literally  quoted  of  adaptations. 
It  would  hardly  be  going  beyond  the  facts  to 
say  that  the  history  of  theories  proves  that 
usually  not  even  preliminary  hypotheses  con- 
cerning causes  are  worked  out  directly  from  an 
analysis  of  effects;  but  the  causes  are  usually 
caught  in  action  during  the  effort  to  discover 
them  inductively,  or  are  reached  in  a  round- 
about way. 


XVI. 

CONCLUSION. 

MUCH  might  be  said  concerning  the  per- 
sonal qualities  of  the  man  that  did  so 
much  scientific  work  of  such  uniformly  high 
character.  The  moral  force  that  overcame  life- 
long physical  suffering,  that  stood  through 
many  years  of  silent  toil  face  to  face  with  the 
certainty  of  abuse  for  its  reward,  that  never 
knew  defeat  and  remained  calm  during  the 
years  of  victory,  has  a  powerful  influence  on 
the  student  of  Darwin.  The  utter  lack  of 
partisanship  for  any  idea,  the  rare  judicial 
temper  that  made  truth  seem  better  than  any 
theory,  the  penetration,  the  power  of  concen- 
tration, the  firm  mental  grasp,  the  inability  to 
leave  anything  unexplained,  —  all  these  high 
qualities  have  their  silent  evidence  in  the  char- 
acter of  Darwin's  scientific  work.  But  his 
intellectual  and  moral  traits  have  been  touched 
upon  here  only  in  so  far  as  was  necessary  in 
order  to  discuss  clearly  his  use  of  the  logical 
processes.  The  effectiveness  of  these  processes 


230  THE  METHOD   OF  DARWIN. 

must  always  be  entirely  dependent  on  the  char- 
acter of  the  individual  using  them. 

Darwin's  views  on  method  can  be  summed 
up  in  the  assertion  that  he  was  afraid  of  every 
statement  or  hypothesis  until  it  was  tested, 
and  indeed  regarded  an  unverified  belief  as 
worthless.  The  starting  points  of  his  investi- 
gations were  frequently  what  seemed  to  other 
men  interesting,  but  unimportant  or  incon- 
venient exceptional  facts.  When  he  sought 
explanations  he  seemed  to  be  trying  to  get  a 
conclusion  by  the  shortest  and  easiest  route, 
with  as  little  labor  as  possible.  But  when  he 
had  once  got  an  hypothesis  he  dragged  it  before 
all  the  multitude  of  facts  that  could  be  made 
to  bear  witness  to  its  truth  or  falsity,  until  it 
seemed  as  if  he  were  trying  to  make  the  inves- 
tigation last  as  long  as  possible.  Time  seemed 
no  longer  worth  considering.  He  always  used 
the  isolated  phenomena  which  were  most  diffi- 
cult to  explain  as  tests  of  the  validity  of  his 
hypotheses.  )/  By  considering  all  possible  objec- 
tions during  the  progress  of  the  development 
of  his  conceptions,  he  threw  a  merciless  light 
on  the  weaknesses  of  his  theories,  and  thus 
gave  them,  in  their  final  form,  as  high  a  degree 
of  probability  as  was  possible.  In  his  treat- 
ment of  negative  evidence  he  never  lost  sight 


CONCLUSION.  231 

of  its  comparatively  small  value,  and  continued 
the  investigation  until,  by  careful  observation 
of  all  collateral  facts,  he  was  able  to  combine 
them  into  a  body  of  positive  evidence  in  sup- 
port of  a  different  or  supplemental  theory. 

Classification  was  with  him  an  invaluable 
instrument  for  extracting  information  from 
bodies  of  facts.  His  works  teem  with  compar- 
ative tables  and  statements  of  results  derived 
from  them.  Analogical  reasoning,  with  all  its 
strength  and  weakness,  was  utilized  as  a  power- 
ful instrument  of  suggestion.  Induction  was 
constantly  active  in  the  formation  of  hypoth- 
eses. He  could  leave  nothing  unexplained. 
He  made  an  hypothesis  for  everything,  and 
then  tested  it  unmercifully  by  deduction.  He 
appreciated  the  immense  importance  of  theory 
to  good  observation,  explained  in  the  light  of 
his  general  theories  great  bodies  of  facts  and 
principles  which  had  been  discovered  empiri- 
cally, and  anticipated  many  important  conse- 
quences of  those  theories.  Whenever  it  was 
possible  he  undertook  to  verify  those  anticipa- 
tions ;  but  did  not  hesitate  to  make  predictions 
that  he  could  not  verify.  And  with  all  his 
vast  and  accurate  knowledge  of  facts  and  his 
logical  power,  he  frequently  fell  into  erroneous 
reasoning. 


232  THE  METHOD   OF  DARWIN. 

The  processes  employed  in  scientific  inves- 
tigations, although  some  of  them  have  been 
treated  in  separate  chapters,  have  a  vital  inter- 
dependence. Darwin  did  not  and  could  not  use 
one  process  until  its  resources  were  exhausted, 
and  then  turn  to  another.  It  was  the  very 
swiftness  with  which  different  processes  were 
successively  brought  to  bear  upon  his  problems 
that  made  it  possible  to  digest  so  thoroughly 
every  set  of  facts  with  which  he  dealt.  What- 
ever may  be  the  future  of  the  particular  con- 
clusions which  Charles  Darwin  reached,  the 
general  method  which  he  employed  and  the 
general  drift  of  his  conclusions  will  have  a 
permanent  value.  All  his  efforts  tended  toward 
the  unification  of  knowledge.  All  his  induc- 
tions became  corollaries  of  one  great  theory; 
all  his  deductions  had  to  do  with  efforts  "to 
test  and  prove  the  truth  of  that  theory.  The 
subordination  of  all  the  devices  of  the  intellect 
to  one  great  comprehensive  purpose  has  given 
a  unity  of  aim  to  all  the  great  works  of  his  life, 
has  made  his  general  method  conspicuously 
lucid,  and  has  knit  the  products  of  his  intellect 
into  one  great  logical  whole. 

THE    END. 


EcfT!B^vN 

IT  KT  T  \r  -r-  T^  „          ^\ 


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TRAVELS.  Translated  by  THOMAS  CARLYLE.  With  Critical 
Introduction  by  EDWARD  DOWDEN,  LL.D.  Edited,  with 
notes,  by  C.  K.  SHORTER.  Portrait.  2  vols. 

PORTRAITS  OF  MEN.  By  C.  A.  SAINTE-BEUVE.  Trans- 
lated by  FORSYTH  EDEVEAIN.  With  Critical  Memoir  by 
WILLIAM  SHARP.  Portrait. 

PORTRAITS  OF  WOMEN.  By  C.  A.  SAINTE-BEUVE.  Trans- 
lated by  HELEN  STOTT.  Portrait. 

NOVALIS  (FRIEDRICH  VON  HARDENBERG).  His  Life, 
Thoughts,  and  Works.  Edited  and  Translated  by  M.  J. 
HOPE. 

THE  COMEDIES  OF  CARLO  GOLDONI.  Edited,  with 
Introduction,  by  HELEN  ZIMMERN. 

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